“Because of FDA rules, supplement manufacturers cannot claim on a label any preventive or therapeutic qualities. For instance, magnesium, vitamin B6, and the herbal treatment chasteberry (Vitex agnus-castus) are proven to help premenstrual syndrome (PMS), but the labels cannot claim this effect. Instead, a label for magnesium might read “helps nerve and muscle function” but not mention PMS, even if that’s the reason you’re purchasing it.”