Vitamin A is normally a fat-soluble vitamin. It occurs in nature as preformed vitamin A and provitamin A (primarily beta carotene). The carotenes are converted into vitamin A in the intestines. Fat-soluble vitamin A can accumulate in the liver, causing side effects, but consuming too much carotene will not result in vitamin A toxicity. Thyroxine, zinc and vitamin E enhance the conversion of carotene to vitamin A. Water-soluble vitamin A results in higher blood levels, with less accumulation in the liver, and, though hard to find, can be used safely in much higher doses.

Dosages of vitamin A below 250,000 IU per day for several months rarely cause toxicity symptoms. Chapped lips and dry skin, which may be early symptoms, will generally occur in the majority of patients, particularly in dry weather.

The first significant toxic symptom is usually headache followed by fatigue, emotional lability, and muscle and joint pain. Laboratory tests appear unreliable for monitoring toxicity, since serum vitamin A levels correlate poorly with toxicity, and SGOT and SGPT are elevated only in symptomatic patients. More importantly, there is concern about the teratogenicity of massive dosages of vitamin A. Women of child-bearing age should use effective birth control during treatment and for at least one month after discontinuation.