Cataracts

• A cataract is a clouding of the lens in the eye that affects vision. Most cataracts are related to aging. Cataracts are very common in older people. The National Eye Institute (NEI) estimates that by age 80, more than half of all Americans either have a cataract or have had cataract surgery. Cataracts generally do not cause surface irritation or pain.
• A cataract can occur in one or both eyes. It cannot spread from one eye to the other.
• Clouded vision can make it more difficult to read, drive a car (especially at night), or to see the expression on another's face. Cataracts commonly affect distance vision and cause problems with glare. The lens is the clear part of the eye that helps to focus light, or an image, on the retina. The retina is the light-sensitive tissue at the back of the eye.
• In a normal eye, light passes through the transparent lens to the retina. Once it reaches the retina, light is changed into nerve signals that are sent to the brain.
• The lens must be clear for the retina to receive a sharp image. If the lens is cloudy from a cataract, the image seen will be blurred.
• Clouding of the lens is a normal part of getting older. The Centers for Disease Control (CDC) has found that approximately half of Americans older than 65 have some degree of clouding of the lens. After age 75, as many as 70% of Americans have cataracts that are significant enough to impair vision. Cataracts occur equally in men and women.
• Most cataracts develop slowly and do not initially disturb eyesight. Cloudiness occurs over time. As the clouding progresses, the cataract eventually interferes with vision.
• In the early stages, stronger lighting and eyeglasses can help the individual deal with the vision problems. But at some point, if impaired vision jeopardizes normal lifestyle, the individual might need surgery. Fortunately, cataract removal is one of the safest, most effective, and most common surgical procedures.

• Normally, the lens of the eye is clear. When a cataract develops, the lens becomes cloudy, similar to a frosted window.
• Located near the front of the eye, the lens focuses light on the retina at the back of the eye. Light passes through the lens to produce a sharp image on the retina. When a cataract forms, the lens can become so opaque and unclear that light cannot easily be transmitted to the retina.
• Often, however, a cataract covers only a small part of the lens and if sight is not greatly impaired, there is no need to remove the cataract. If a large portion of the lens becomes cloudy, sight can be partially or completely lost until the cataract is removed.
• Cataracts do not spread from eye to eye, though it may develop in both eyes at the same time. There are many common misconceptions about cataracts. A cataract is not a film visible on the outside of the eye, is not caused from overuse of the eyes, and using the eye when reading, watching TV, or using a computer does not make it worse. Cataracts usually develop gradually over many years; rarely, they may develop over a few months. Having a cataract does not mean an individual will be permanently blind. Cataracts lead to blindness if left untreated.
• Depending on the size and location of the cloudy areas in a lens, an individual may or may not be aware that a cataract is developing. If the cataract is located on the outer edge of the lens, no change may be noticed in vision, but if the cloudiness is located near the center of the lens, it usually interferes with clear sight. As cataracts develop, there may be hazy, fuzzy and blurry vision. Double vision may also occur when a cataract is beginning to form. The eyes may be more sensitive to light and glare, making night driving difficult.
• With cataracts, there may be a need to change eyeglass prescriptions frequently. As the cataract worsens, stronger glasses are no longer able to improve sight. It may help to hold objects closer to the eye when reading or doing close-up work. The pupil, which is normally black, may undergo noticeable color changes and appear to be yellowish or white.
• Nuclear cataract: A nuclear cataract is most commonly seen as it forms. This cataract forms in the nucleus, the center of the lens, and is due to natural aging changes.
• Cortical cataract: A cortical cataract, which forms in the lens cortex, gradually extends its spokes from the outside of the lens to the center. Many diabetics develop cortical cataracts.
• Subcapsular cataract: A subcapsular cataract begins at the back of the lens. Individuals with diabetes, high farsightedness, retinitis pigmentosa, or those taking high doses of steroids may develop a subcapsular cataract.
• Other types of cataracts: Although most cataracts are related to aging, there are other types of cataracts including secondary, traumatic, congenital, and radiation cataracts.
• Secondary cataract:
  Cataracts can form after surgery for other eye problems, such as glaucoma. Cataracts also can develop in people who have other health problems, such as diabetes. Cataracts are sometimes linked to steroid use.
• Traumatic cataract: Cataracts can develop after an eye injury, sometimes years later.
• Congenital cataract: Congenital is defined as being present at birth. Some babies are born with cataracts or develop them in childhood, often in both eyes. These cataracts may be so small that they do not affect vision. If they do, the lenses may need to be removed surgically. Congenital cataracts are uncommon. The causes of congenital cataracts include German measles (rubella) during pregnancy.
• Radiation cataract: Cataracts can develop after exposure to certain types of radiation, including gamma radiation.

• All individuals are at risk of developing cataracts due to the aging process. Age is the single greatest risk factor for the development of cataracts. By age 65, about half of all Americans have developed some degree of lens clouding, although it may not impair vision.
• Other factors that increase the risk of cataracts include: diabetes; family history of cataracts; previous eye injury or inflammation; previous eye surgery; prolonged use of corticosteroids, such as prednisone (Deltasone®); excessive exposure to sunlight; exposure to ionizing radiation; and smoking.

• A cataract usually develops slowly and causes no pain. At first, the cloudiness may affect only a small part of the lens and the individual may be unaware of any vision loss. Over time, however, as the cataract grows larger, it clouds more of the lens. When significantly less light reaches the retina, vision becomes impaired.
• Symptoms of a cataract include: clouded, blurred, or dim vision; increasing difficulty with vision at night; sensitivity to light and glare; halos around lights; the need for brighter light for reading and other activities; frequent changes in eyeglass or contact lens prescription; fading or yellowing of colors; and double vision in a single eye.
• A cataract is not dangerous to the physical health of the eye unless the cataract becomes completely white, a condition known as an overripe (hypermature) cataract. This can cause inflammation, pain, and headache. A hypermature cataract is very uncommon, but it requires surgical removal if it is associated with inflammation or pain.

• The only way to know for sure if an individual has a cataract is to have an eye examination that includes several tests, including visual acuity, slit-lamp examination, and a retinal examination.
• Visual acuity test: Acuity refers to the sharpness of vision or how clearly the individual sees an object. In this test, the eye doctor checks to see how well the individual reads letters from across the room. The eyes are tested one at a time, while the other eye is covered. Using a chart (called a Snellen eye chart) with progressively smaller letters from top to bottom, an eye doctor determines if the individual has 20/20 vision or less acute vision.
• Slit-lamp examination: A slit lamp allows the eye doctor to see the structures at the front of the eye under magnification. The microscope is called a slit lamp because it uses an intense line of light (or a slit) to illuminate the cornea, iris, lens, and the space between the iris and cornea. The slit allows the doctor to view these structures in small sections, which makes it easier to detect any small abnormalities.
• Retinal examination: In a retinal examination, an eye doctor puts dilating drops in the eyes to open the pupils wide and provide a bigger window to the back of the eyes. Using a slit lamp or a special device called an ophthalmoscope, the doctor can examine the lens for signs of a cataract and, if needed, determine the density of the clouding. An eye doctor will also check for glaucoma and, if the individual has blurred vision or discomfort, the doctor will check for other problems involving the retina and the optic nerve. Dilating drops usually keep the pupils open for a few hours before their effect gradually wears off. Until then, the individual will probably have difficulty focusing on close objects, while distance vision is generally less affected. With the pupils open this wide, individuals may want sunglasses for their trip home, especially if it is a bright day. Also, it may be safer to let someone else do the driving.

• In rare cases, there can be complications to cataract surgery such as loss of vision, bleeding, double vision, and infection. Inflammation and fluctuating eye pressure can be a side effect of this surgery as well. While instances of side effects are documented, it should be said that they happen rarely.
• Retinal detachment: Retinal detachment is a condition that occurs when fluid seeps through a tear in the retina. The seepage causes the retina to detach from the back of the eye. While cataract surgery is not the only cause of retinal detachment, it occurs in approximately 0.5% of cataract surgery patients. Retinal detachment may be caused by trauma, advanced diabetes, or an inflammatory disorder. Retinal detachment also occurs in individuals who have had previous eye surgeries and who are extremely nearsighted. Symptoms of retinal detachment are flashes of light or dark spots in the field of vision. Some of these symptoms occur naturally after cataract surgery, but individuals who experience them should contact their doctors immediately. Another symptom that requires immediate medical attention is a shadow that seems to move across part of or the entire field of vision.
• Cystoid macula edema: Cystoid macula edema (swelling) is decreased vision in the central part of the visual field due to swelling in the layer of nerve cells that covers the entire back part of the eye, called the retina. The macula is the part of the retina that responds to light in the central part of the visual field. After cataract surgery that has had no complications, the blood vessels in the retina can swell and leak. As the fluid accumulates, the macula may swell. As time progresses after cataract surgery and the individual notices decreased vision, they should contact a doctor and tests can be done, such as ocular coherence tomography, to determine the extent of the swelling.
• Endophthalmitus: Endophthalmitus is an infection inside of the eye. Symptoms include pain and excessive redness and swelling, sensitivity to light, and perhaps loss of vision. Usually, these symptoms reveal themselves within the first few days after surgery. Antibiotic eye drops are administered the day of surgery to prevent infection. In spite of this, one in 3,000 patients undergoing cataract surgery develops endophthalmitis.
• Choroidal hemorrhage: Choroidal hemorrhage is when the choroid, the web of fine blood vessels that supplies blood to the retina, begins to bleed during surgery. It usually occurs in older individuals or individuals who have high blood pressure or glaucoma. A hemorrhage confined to a small area will have very little visual loss but if the hemorrhage is severe, then significant visual loss may occur. Modern micro surgical techniques rely on small incisions; the severity of hemorrhages has reduced dramatically.
• Secondary cataracts: Secondary cataracts develop years after cataract surgery. A secondary cataract results in the clouding of the back of the lens capsule. This is the part of the lens that wasn't removed during the first surgery and that supports the lens implant (IOL). This condition can also be called 'aftercataracts' and posterior capsule opafication. The problem is treated quickly and simply with a laser called yttrium-aluminum-garnet, or YAG for short. It is a quick and painless procedure that usually takes less than five minutes to complete. The individual can resume normal activity within hours.
• Blindness: According to estimates, age-related cataracts are responsible for 48% of world blindness, which represents about 18 million people. Cataracts are a leading cause of blindness among older adults in the United States.

• The symptoms of an early cataract may be improved with eyeglasses, brighter lighting, anti-glare sunglasses, or magnifying lenses. If these measures do not help, surgery is the only effective treatment. Surgery involves removing the cloudy lens and replacing it with an artificial lens.
• A cataract needs to be removed only when vision loss interferes with everyday activities, such as driving, reading, or watching TV. The individual and their eye care professional can make this decision together. Once the individual understands the benefits and risks of surgery, they can make an informed decision about whether cataract surgery is right. In most cases, delaying cataract surgery will not cause long-term damage to the eye or make the surgery more difficult.
• Cataract removal is one of the most common operations performed in the United States. It also is one of the safest and most effective types of surgery. In about 90% of cases, people who have cataract surgery have better vision afterward. Approximately 2.7 million Americans undergo cataract surgery.
• If an individual needs cataracts removed from both eyes, surgery usually will be done on only one eye at a time. An uncomplicated surgical procedure lasts only about 10 minutes. However, the individual may be in the outpatient facility for 90 minutes or longer because extra time will be needed for preparation and recovery. At least a few days to weeks typically will be needed between surgeries, so that the first eye has the chance to heal and be evaluated in a follow-up exam for any possible problems.
• Due to improvements in technology, most eye doctors use cataract surgery at an earlier stage of development rather than waiting until the cataract is advanced and very dense.
• There are two types of cataract surgery. An eye doctor can explain the differences and help determine which is best.
• Sometimes a cataract should be removed even if it does not cause problems with vision. For example, if the individual has diabetes, it may be best to remove a cataract before vision is affected.
• Phacoemulsification, or phaco: During phacoemulsification surgery, a small incision is made on the side of the cornea, the clear, dome-shaped surface that covers the front of the eye. An eye doctor inserts a tiny probe into the eye. This device emits ultrasound waves that soften and break up the lens so that it can be removed by suction. Most cataract surgery today is done by phacoemulsification, also called "small incision cataract surgery."
• Extracapsular surgery: During extracapsular surgery, an eye doctor makes a longer incision on the side of the cornea and removes the cloudy core of the lens in one piece. The rest of the lens is removed by suction.
• After the natural lens has been removed, it often is replaced by an artificial lens, called an intraocular lens (IOL). An IOL is a clear, plastic lens that requires no care and becomes a permanent part of the eye. Light is focused clearly by the IOL onto the retina, improving vision. The individual will not feel or see the new lens.
• Some individuals cannot have an IOL. They may have another eye disease or have problems during surgery. For these patients, soft contact lens, or glasses that provide high magnification, may be suggested.
• Risks of cataract surgery: As with any surgery, cataract surgery poses risks, such as infection and bleeding. Before cataract surgery, the doctor may ask the individual to temporarily stop taking certain medications that increase the risk of bleeding during surgery, such as aspirin, warfarin (Coumadin®), and dietary supplements such as ginger (Zingiber officinalis)
  and garlic (Allium sativum). After surgery, the individual must keep the eye clean, wash their hands before touching the eye, and use the prescribed medication, including antibiotics to help minimize the risk of infection and anti-inflammatory drugs to decrease inflammation. Serious infection can result in loss of vision. Other risks include retinal detachment and cystoid macula edema (see the complications section of this monograph for more information).
• What to expect before, during, and after surgery: A week or two before surgery, an eye doctor will perform some tests, including measuring the curve of the cornea and the size and shape of the eye. This information helps the doctor choose the right type of IOL. The individual may be asked not to eat or drink anything 12 hours before surgery.
• At the hospital or eye clinic, drops will be put into the eyes to dilate the pupil. The area around the eye will be washed and cleansed.
• The operation usually lasts less than one hour and is almost painless. Many individuals choose to stay awake during surgery. Others may need to be put to sleep for a short time. If the individual is awake, they will have an anesthetic to numb the nerves in and around the eye.
• After the operation, a patch may be placed over the eye. The individual will be told to rest for a while. A medical team will watch for any problems, such as bleeding. Most individuals who have cataract surgery can go home the same day. The individual will need someone to drive them home.
• Itching and mild discomfort are normal after cataract surgery. Some fluid discharge is also common. The eye may be sensitive to light and touch. After one or two days, moderate discomfort should disappear. Eyes drops for itching may be prescribed by a doctor.
• For a few days after surgery, a doctor may ask the individual to use antibiotic eyedrops to help healing and decrease the risk of infection, such as ciprofloxacin (Cipro®). Also, ketorolac (Toradol®) eye drops may be prescribed to be placed in the eye to decrease inflammation. The individual will need to wear an eye shield or eyeglasses to help protect the eye and avoid rubbing or pressing on the eye. When the individual is at home, it is recommended by healthcare professionals to try not to bend from the waist to pick up objects on the floor and not to lift any heavy objects. Straining and bending may cause pressure in the eyes and damage the eye. The individual can walk, climb stairs, and do light household chores.
• In most cases, healing will be complete within eight weeks. A doctor will schedule exams to check on the progress.
• Individuals can normally return quickly to many everyday activities, but their vision may be blurry. The healing eye needs time to adjust so that it can focus properly with the other eye, especially if the other eye has a cataract. The individual must ask a doctor when driving can be resumed. Headaches, dizziness, and blurry vision may occur.
• If an IOL was placed in the eye, colors may appear very bright. The IOL is clear, unlike the natural lens that may have had a yellowish/brownish tint. Within a few months after receiving an IOL, the individual will become accustomed to improved color vision. Also, when the eye heals, new glasses or contact lenses may be needed.
• If sight has been lost from cataract or cataract surgery, it is important to ask an eye care professional about low vision services and devices that may help improve the remaining vision. Ask for a referral to a specialist in low vision. Many community organizations and agencies offer information about low vision counseling, training, and other special services for people with visual impairments. A nearby school of medicine or optometry may provide low vision services.

B

Good scientific evidence

• Coleus
  : Some evidence suggests that coleus improves glaucoma. More studies are needed. Coleus is generally regarded as safe, although long-term safety data are lacking. Avoid in individuals with a known allergy or hypersensitivity to Coleus forskohlii and related species. Rash may occur in sensitive individuals. Inhalation of forskolin may cause sore throat, upper respiratory tract irritation, mild to moderate cough, tremor, or restlessness. Coleus eye drops may produce a milky covering over the eyes. Use cautiously in patients with thyroid disorders, diabetes, low blood pressure, heart disease, asthma, or a history of bleeding, hemostatic disorders, or drug-related hemostatic problems. Discontinue use in patients at least two weeks prior to surgical or dental procedure, due to risk of bleeding. Avoid use in patients with active bleeding. Avoid during pregnancy due to possibility of abortifacient (abortion inducing) effects.

C

Unclear or conflicting scientific evidence

• Alpha-lipoic acid
  : There are some human studies of alpha-lipoic acid (ALA) as a treatment for glaucoma, but there is not enough scientific evidence to make a conclusion at this time. Avoid if allergic to ALA. Use cautiously with diabetes and thyroid diseases. Avoid with thiamine deficiency or alcoholism. Avoid if pregnant or breastfeeding.
• Berberine
  : Preliminary study indicates that berberine does not appear to reduce intraocular pressure in patients with glaucoma. The safety and effectiveness of berberine for this indication remains unclear. Additional study is needed in this area.
  Avoid if allergic or hypersensitive to berberine, to plants that contain berberine, or to members of the Berberidaceae family. Avoid in newborns due to potential for increase in free bilirubin, jaundice, and development of kernicterus. Use cautiously with cardiovascular disease, gastrointestinal disorders, hematologic disorders, leucopenia, kidney disease, liver disease, respiratory disorders, cancer, hypertyraminemia, diabetes, or hypotension. Use cautiously in children due to lack of safety information. Use cautiously in individuals with high exposure to sunlight or artificial light. Use cautiously for longer than eight weeks due to theoretical changes in bacterial gut flora. Use cautiously if taking anticoagulants, antihypertensives, sedatives, anti-inflammatories, medications metabolized by CYP P450 3A4 including cyclosporin, or any prescription medications. Avoid if pregnant or breastfeeding.
• Beta-carotene
  : Beta-carotene is a member of the carotenoids, which are highly pigmented (red, orange, yellow), fat-soluble compounds naturally present in many fruits, grains, oils, and vegetables (green plants, carrots, sweet potatoes, squash, spinach, apricots, and green peppers). Study results of beta-carotene supplementation for cataract prevention are conflicting. Further well-designed clinical trials are needed before a conclusion can be drawn. Supplemental beta-carotene may increase the risk of lung cancer, prostate cancer, intracerebral hemorrhage, and cardiovascular and total mortality in people who smoke cigarettes or have a history of high-level exposure to asbestos. Beta-carotene from foods does not seem to have this effect. In those who smoke, beta-carotene may increase cardiovascular death, including heart attack. Avoid if sensitive to beta-carotene, vitamin A or any other ingredients in beta-carotene products.
• Bilberry
  : Bilberry (Vaccinium myrtillus) is a close relative of blueberry. Bilberry extract has been used for a number of eye problems, including the prevention of cataracts. Products containing bilberry may also reduce the risk for developing glaucoma. At this time, there is limited scientific information in this area. Additional study is needed. Bilberry may increase bleeding in sensitive individuals, such as those taking blood thinning medications including warfarin (Coumadin®) and aspirin. Avoid if allergic to plants in the Ericaceae family or to anthocyanosides (a component of bilberry). Avoid with a history of low blood pressure, heart disease, bleeding, diabetes, blood clots, or stroke. Avoid if pregnant or breastfeeding. Stop use before surgeries or dental or diagnostic procedures involving blood tests.
• Danshen
  : Danshen may be beneficial in glaucoma therapy, but further studies are needed in humans before a clear conclusion can be drawn. Danshen should not be used in place of more proven therapies, and patients with glaucoma should be evaluated by a qualified eye care specialist. Avoid if allergic or hypersensitive to danshen. Use cautiously with altered immune states, arrhythmia, compromised liver function or a history of glaucoma, stroke, or ulcers. Stop use two weeks before surgery/dental/diagnostic procedures with bleeding risk, and do not use immediately after these procedures. Use cautiously if driving or operating heavy machinery. Avoid if taking blood thinners (anticoagulants), digoxin, or hypotensives, including ACE inhibitors such as captopri, or Sophora subprostrata root or herba serissae. Avoid with bleeding disorders, low blood pressure and following cerebal ischemia. Avoid if pregnant or breastfeeding.
• Ginkgo
  : It is not clear if ginkgo may improve intraocular pressure and blood flow in patients with glaucoma. Some study results conflict or have not been significant. Further research is needed before a conclusion can be made.
• Avoid if allergic or hypersensitive to members of the Ginkgoaceae
  family.
  If allergic to mango rind, sumac, poison ivy or oak or cashews, then allergy to ginkgo is possible. Avoid with blood-thinners (like aspirin or warfarin (Coumadin®)) due to an increased risk of bleeding. Ginkgo should be stopped two weeks before surgical procedures. Ginkgo seeds are dangerous and should be avoided. Skin irritation and itching may also occur due to ginkgo allergies. Do not use ginkgo in supplemental doses if pregnant or breastfeeding.
• Kinetin
  : Kinetin is a chemical analogue of cytokinins, a class of plant hormones that promotes cell division. Kinetin is found in both plants and animals. Side effects of surgery for cataracts may include pain, infection, swelling, bleeding, or retinal detachment. The use of kinetin during cataract surgery may lower adverse effects associated with cataracts. More research is needed in this area. Avoid if allergic or hypersensitive to kinetin. Use cautiously with coagulation or hematologic (blood) disorders or taking anticoagulants or antiplatelets (blood thinners). Avoid if pregnant or breastfeeding.
• Kudzu
  : In China, the main herb-derived eye drops for glaucoma are pueraria flavonoids. The addition of puerarin to conventional drugs for glaucoma have yielded favorable results. Additional research is needed to confirm these results. Use caution with anticoagulants/anti-platelet and blood pressure lowering agents, hormones, antiarrhythmics, benzodiazepines, bisphosphonates, diabetes medications, drugs that are metabolized by the liver's cytochrome P450 enzymes, mecamylamine, neurologic agents, and methotrexate. Well designed studies on the long-term effects of kudzu are currently unavailable. Avoid if allergic or hypersensitive to Pueraria lobata or to members of the Fabaceae/Leguminosae family. Avoid if pregnant or breastfeeding.
• Lutein
  : Lutein and zeaxanthin are found in high levels in foods such as green vegetables, egg yolk, kiwi fruit, grapes, orange juice, zucchini, squash, and corn. For some commercially available supplements, lutein is extracted from marigold petals. Human study has not found a benefit of lutein supplementation on visual performance in people with cataracts. More research is needed. Avoid if allergic or hypersensitive to lutein or zeaxanthin. Use cautiously if at risk for cardiovascular disease or cancer. Avoid if pregnant or breastfeeding.
• Melatonin
  : It has been theorized that high doses of melatonin may increase intraocular pressure and the risk of glaucoma, age-related maculopathy and myopia, or retinal damage. However, there is preliminary evidence that melatonin may actually decrease intraocular pressure in the eye, and it has been suggested as a possible therapy for glaucoma. Additional study is necessary in this area. Patients with glaucoma taking melatonin should be monitored by a healthcare professional. Melatonin is not to be used for extended periods of time. Interactions with drugs may occur.
• Riboflavin (vitamin B2)
  : Riboflavin is a water-soluble vitamin that is involved in vital metabolic processes in the body, and is necessary for normal cell function, growth, and energy production. Small amounts of riboflavin are present in most animal and plant tissues. It has been suggested that low riboflavin levels may be a risk factor for developing cataracts, and that riboflavin supplementation may be beneficial for cataract prevention. Additional evidence is needed before a clear conclusion can be drawn.
  Avoid if allergic or hypersensitive to riboflavin. Since the amount of riboflavin a human can absorb is limited, riboflavin is generally considered safe. Riboflavin is generally regarded as safe during pregnancy and breastfeeding. The U.S. Recommended Daily Allowance (RDA) for riboflavin in pregnant women is higher than for non-pregnant women. For pregnant women, the RDA is 1.4 milligrams daily; for breastfeeding women, the RDA is 1.6 milligrams.
• Selenium
  : Selenium is a trace mineral found in soil, water, and some foods. It is an essential element in several metabolic pathways. Preliminary research reports that selenium supplementation may affect the development of cataracts. Further research is needed before a clear conclusion can be drawn.
  Avoid if allergic or sensitive to products containing selenium. Avoid with a history of nonmelanoma skin cancer. Selenium is generally regarded as safe for pregnant or breastfeeding women. However, animal research reports that large doses of selenium may lead to birth defects.
• Thiamin
  : Thiamin, also known as thiamine and vitamin B1, is a water-soluble B-complex vitamin. Preliminary evidence suggests that high dietary thiamin intake may be associated with a decreased risk of cataracts. Further evidence is necessary before a firm conclusion can be reached concerning the use of thiamin for cataract prevention.
  Thiamin is generally considered safe and relatively nontoxic. Avoid if allergic or hypersensitive to thiamin.
  Rare hypersensitivity/allergic reactions have occurred with thiamin supplementation. Skin irritation, burning, or itching may rarely occur at injection sites. Large doses may cause drowsiness or muscle relaxation. Use supplemental doses cautiously if pregnant or breastfeeding.
• Thymus extract
  : Well-designed clinical trials are required before thymus extract can be recommended in the treatment of glaucoma. Avoid if allergic or hypersensitive to thymus extracts. Use bovine thymus extract supplements cautiously due to potential for exposure to the virus that causes "mad cow disease." Avoid use with an organ transplant or other forms of allografts or xenografts. Avoid if receiving immunosuppressive therapy, with thymic tumors, myasthenia gravis (neuromuscular disorder), untreated hypothyroidism, or if taking hormonal therapy. Avoid if pregnant or breastfeeding; thymic extract increases human sperm motility and progression.
• Vitamin A
  : Vitamin A is a fat-soluble vitamin, which is derived from two sources: preformed retinoids and provitamin carotenoids. Retinoids such as retinal and retinoic acid are found in animal sources such as liver, kidney, eggs, and dairy produce. Carotenoids like beta-carotene (which has the highest vitamin A activity) are found in plants such as dark or yellow vegetables and carrots. There is currently insufficient evidence to form a clear conclusion regarding use of Vitamin A for cataract prevention. Vitamin A should only be used within the recommended dietary allowance, as toxicity can occur if taken at high dosages. Vitamin A excess, as well as deficiency, have been associated with birth defects. Excessive doses of vitamin A have been associated with central nervous system malformations.
  Avoid if allergic or hypersensitive to vitamin A. Use cautiously with liver disease or alcoholism. Smokers who consume alcohol and beta-carotene may be at an increased risk for lung cancer or heart disease. Use cautiously if breastfeeding because the benefits or dangers to nursing infants are not clearly established.
• Vitamin E
  : Vitamin E is a fat-soluble vitamin with antioxidant properties. There is conflicting evidence regarding the use of vitamin E to prevent cataracts. Although some studies across populations have suggested some effects for cataract prevention (which may take up to 10 years to yield benefits), other studies in humans report a lack of benefits when used either alone or in combination with other antioxidants. Additional research is necessary before a clear conclusion can be reached. Vitamin E may increase bleeding in sensitive individuals, such as those taking blood thinning medications including warfarin (Coumadin®) and aspirin.
  Avoid if allergic or hypersensitive to vitamin E. Avoid with retinitis pigmentosa (loss of peripheral vision). Avoid above the recommended daily level in pregnant women and breastfeeding women.

D

Fair negative scientific evidence

• Vitamin C
  :
  Although early population research suggested a reduction in cataract formation among individuals taking vitamin C, subsequent research did not find effects for cataract prevention or progression. Avoid if allergic or sensitive to vitamin C product ingredients. Vitamin C is generally considered safe in amounts found in foods. Vitamin C supplements are also generally considered safe in most individuals if taken in recommended doses. Avoid high doses of vitamin C with glucose 6-phosphate dehydrogenase deficiency, kidney disorders or stones, cirrhosis (inflammation of the liver), gout, or paroxysmal nocturnal hemoglobinuria (bleeding disorder). Vitamin C intake from food is generally considered safe if pregnant or breastfeeding. It is not clear if vitamin C supplements in doses higher than Dietary Reference Intake recommendations are safe for pregnant or breastfeeding women. Vitamin C is naturally found in breast milk.