From vitamins C and D to calcium and magnesium, it’s critical to know whether you’re taking the correct dosage to avoid health problems

There are more than 100,000 supplements on the US market – capsules, powders, tablets and gummies sold to improve or maintain health. Supplements can contain vitamins, minerals, botanicals and amino acids on their own or in various combinations.

The consumption of these products is surging. But it’s a common misunderstanding that these products are entirely safe, says Dr Pieter Cohen, an internist and associate professor at Harvard Medical School. Excessive amounts of nutrients can cause health problems, so it’s critical to know whether you’re using the correct dosage of high-quality products.

This article was amended on 9 January 2026 to clarify the possible negative side effects of probiotics.