If you are being treated with any of the following medications, you should not use vitamin A without first talking to your health care provider.

Tetracycline antibiotics -- People who take a type of antibiotics called tetracyclines and also take high doses of vitamin A may be at risk for a condition called intracranial hypertension, a rise in the pressure of brain fluid. Tetracyclines include:

• Demeclocycline (Declomycin)
• Minocycline (Minocin)
• Tetracycline (Achromycin)

Antacids -- One study suggests that the combination of vitamin A and antacids may be more effective than antacids alone in healing ulcers.

Anticoagulants (blood thinners) -- Long-term use of vitamin A or taking high doses may increase the risk of bleeding for those taking blood-thinning medications, particularly warfarin (Coumadin). Talk to your doctor before taking vitamin A.

Cholesterol-lowering medications (bile acid sequestrants) -- The medications cholestyramine (Questram) and colestipol (Colestid) may reduce the body's ability to absorb vitamin A and lead to lower levels in the body. A water-soluble form of vitamin A may help. Another class of cholesterol-lowering medications called statins may actually increase vitamin A levels in the blood.

Doxorubicin -- Test tube studies suggest that vitamin A may enhance the action of doxorubicin, a medication used to treat cancer. More research is needed, however, to know whether this has any practical application. If you are undergoing treatment for cancer, talk to your oncologist before taking vitamin A or any supplement.

Neomycin (Mycifradin) -- This antibiotic may reduce the body's ability to absorb vitamin A, especially when taken in large doses.

Omeprazole -- Omeprazole (used for gastroesophageal reflux disease or "heartburn") may influence the absorption and effectiveness of beta-carotene supplements. It is not known whether this medication affects the absorption of beta-carotene from foods.

Retinoids -- These medications are a synthetic form of vitamin A and are sometimes prescribed in high doses. People who take retinoids should not take any additional vitamin A supplements. In addition, these drugs can cause severe birth defects. Women of child-bearing age must have two negative pregnancy tests and be on two forms of birth control before taking these medications. Anyone taking retinoids will be monitored closely by their doctor. Retinoids include:

• Acitretin (Soriatane)
• Bexarotene (Targretin)
• Isotretinoin (Accutane)
• Tazarotene (Avage)

Tretinoin (Retin-A) is usually prescribed for topical use to treat acne or reduce wrinkles and is not as concentrated as other retinoids. However, it is still a good precaution to avoid taking a vitamin A supplement while using Retin-A.

Orlistat (Alli) and Olestra -- Orlistat, a medication used for weight loss, and olestra, a substance added to certain foods, both prevent the body from absorbing fat and calories. They may also prevent the body from absorbing enough vitamin A. The Food and Drug Administration requires that vitamin A and other fat-soluble vitamins (namely, D, E, and K) be added to food products containing olestra. In addition, people who take either prescription orlistat or over-the-counter Alli may want to take a multivitamin.