Schulz expressed concern about our statement that, when women of childbearing potential, persons with liver disease, and heavy consumers of alcohol are excluded, vitamin A is safe in dosages of up to 100 000 IU/d.1 We respond to Schulz with the following points.
First, to our knowledge, no reports are available that have prospectively evaluated the safety of vitamin A doses in excess of 100 000 IU/d. Therefore, we cannot say with absolute certainty that megadoses of vitamin A are safe.
Second, yet, others2 who have extensively reviewed the epidemiologic literature have noted that long-term vitamin A toxic effects are rare at doses less than 100 000 IU/d. Importantly, not only dose, but also duration and pre-existing comorbidity, influence the occurrence of toxic reactions to vitamin A.
Third, for the above reasons, toxic effects have been observed at doses as low as 4000 IU/d.3 But, toxic effects
Meyers DG, Maloley PA, Weeks D. Comments on the Safety of Antioxidant Vitamin Supplementation-Reply. Arch Intern Med. 1997;157(15):1777. doi:10.1001/archinte.1997.00440360222038