Medically reviewed by Jamie Johnson, RDN If you've recently started taking a new vitamin or supplement, you might wonder how long it takes for vitamins to work​ or when you’ll notice a difference. The ...

Women who might become pregnant should be especially cautious with vitamin A supplements or avoid them entirely .

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 ...