Dr. Keith Roach, syndicated columnist

DEAR DR. ROACH: If I am taking vitamin A, what is a reasonable dose? I have seen reports that taking too much can cause liver damage, but nobody says how much is a problem. — M.E.B.

ANSWER: I think many media sources avoid the complexity of the issue. The preferred unit to describe doses of vitamin A are now micrograms RAE, or “retinol activity units.” Different vitamin A preparations, such as retinol, alpha- and beta-carotene, have different RAE per IU.

For example, 3,000 IU of vitamin A (as retinol) is 900 mcg RAE. About 18,000 IU beta carotene, which is less potent than retinol, in food is also 900 mcg RAE, which is the recommended daily allowance (RDA) of vitamin A for adult men. (It’s 700 mcg for adult women.)

Vitamin A deficiency is rare in North America, except in people who cannot absorb fat well. This includes people with untreated celiac disease; some people with severe Crohn’s disease of the small intestine; people with severe pancreas disease; and especially those who have undergone bariatric surgery. The correct dose of vitamin A in these populations needs to be individually determined by checking blood levels during treatment.

In healthy individuals, vitamin A supplementation is not necessary, and toxicity (including liver disease) almost always comes from people who are taking supplemental vitamin A. As a general rule, 10 times the RDA taken chronically is enough to cause toxicity. Roughly speaking, this is about 30,000 IU. (I still see vitamin A sold at vitamin shops and online most often as IU.)

Symptoms of vitamin A excess include dry skin, headache, fatigue and nausea. I found doses of 25,000 available, which is perilously close to the toxic dose.

Women who might become pregnant should be especially cautious with vitamin A supplements or avoid them entirely (except in special circumstances like those who cannot absorb it well). A dose just over the daily value of 3,000 mcg RAE or 10,000 IU of retinol can cause fetal loss or severe birth defects. Prenatal vitamins do not contain excess vitamin A but should not be taken in combination with other vitamin A supplements.

There has been widely spread misinformation about vitamin A and measles. In people with severe vitamin A deficiency (mostly in South Asia and sub-Saharan Africa), measles repletion does reduce the risk of death from measles. But vitamin A is not an effective treatment or a preventive of measles when vitamin A deficiency is unlikely.

DEAR DR. ROACH: I read a few years ago that tanezumab was to be approved soon. What happened? I was looking forward to it to help my arthritis. — J.K.

ANSWER: Unfortunately, as it often happens with promising drugs, the drug was not approved by the Food and Drug Administration. Tanezumab blocked a chemical in the brain called nerve growth factor, which led to reduced pain and improved function. But in clinical trials, some people developed rapidly progressive osteoarthritis.

Because of this, the FDA stated the drug companies had not proven that the benefits outweigh the risks. The drug companies who were jointly developing this drug have reportedly stopped any further development.

EDITOR’S NOTE: Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu.