Vitamin A (retinol, retinoic acid) is a nutrient important to vision, growth, cell division, reproduction and immunity. Vitamin A also has antioxidant properties. Antioxidants are substances that might protect your cells against the effects of free radicals — molecules produced when your body breaks down food or is exposed to tobacco smoke and radiation. Free radicals might play a role in heart disease, cancer and other diseases.

Vitamin A is found in many foods, such as spinach, dairy products and liver. Other sources are foods rich in beta-carotene, such as green leafy vegetables, carrots and cantaloupe. Your body converts beta-carotene into vitamin A.

As an oral supplement, vitamin A mainly benefits people who have a poor or limited diet or who have a condition that increases the need for vitamin A, such as pancreatic disease, eye disease or measles. If you take vitamin A for its antioxidant properties, keep in mind that the supplement might not offer the same benefits as naturally occurring antioxidants in food.

The recommended daily amount of vitamin A is 900 micrograms (mcg) for adult men and 700 mcg for adult women.

Research on oral vitamin A for specific conditions shows:

• Acne. Large doses of oral vitamin A supplements don't appear to affect acne.
• Age-related macular degeneration. A large clinical trial showed that people at high risk of advanced age-related macular degeneration reduced their risk of developing the condition by 25 percent by taking a specific combination of vitamins that included beta-carotene. It's not entirely clear what role beta-carotene played.
• Cancer. The association between use of vitamin A supplements and reduced risk of lung, prostate and other types of cancer is unclear.
• Measles. Vitamin A supplements are recommended for children with measles who are at an increased risk of vitamin A deficiency. Research suggests that supplementation might reduce death due to measles.
• Vitamin A deficiency. People who have low levels of vitamin A appear to benefit most from vitamin A supplements. This kind of deficiency isn't common in the United States. Vitamin A deficiency causes anemia and dry eyes.

Beyond use as an oral supplement, vitamin A is used in topical creams to reduce fine wrinkles, splotches and roughness and treat acne.

Too much vitamin A can be harmful. Even a single large dose — over 200,000 mcg — can cause:

• Nausea
• Vomiting
• Vertigo
• Blurry vision

Taking more than 3,000 mcg a day of oral vitamin A supplements long term can cause:

• Bone thinning
• Liver damage
• Headache
• Diarrhea
• Nausea
• Skin irritation
• Pain in the joints and bone
• Birth defects

If you are or might become pregnant, talk to your doctor before taking vitamin A. Excess use of vitamin A during pregnancy has been linked to birth defects.

Possible interactions include:

• Anticoagulants. Oral use of vitamin A supplements while taking these medications used to prevent blood clots might increase your risk of bleeding.
• Bexarotene (Targretin). Taking vitamin A supplements while using this topical cancer drug increases the risk of the drug's side effects, such as itchy, dry skin.
• Hepatotoxic drugs. Taking high doses of vitamin A supplements can cause liver damage. Combining high doses of vitamin A supplements with other drugs that can damage the liver could increase the risk of liver disease.
• Orlistat (Alli, Xenical). This weight-loss drug can decrease the absorption of food sources of vitamin A. Your doctor might suggest that you take a multivitamin with vitamin A and beta-carotene while taking this medication.
• Retinoids. Don't use vitamin A supplements and these oral prescription drugs at the same time. This could increase the risk of high vitamin A blood levels.