This is the single most important thing to understand about supplements. NIH's Office of Dietary Supplements states that medicines must be approved by FDA before they can be sold, but supplements do not require this approval, and FDA does not determine whether supplements are effective before they're marketed. Companies are responsible for their own evidence of safety and truthful labeling. FDA acts mostly after the fact — pulling products or requesting recalls once a problem surfaces.

FDA warns that some products marketed as supplements are contaminated with ingredients not listed on the label — often prescription drug ingredients, banned substances or harmful chemicals — and that some manufacturers intentionally hide them to get around FDA review. Be especially wary of products sold for sexual enhancement, energy, weight loss, pain, bodybuilding or sleep. FDA cannot test these products before they enter the market, and its health-fraud database “covers only a small fraction” of contaminated products.

A 2023 JAMA analysis bought 25 melatonin gummy products and measured what was actually in them. The real melatonin content ranged from 74% to 347% of the labeled amount — only 3 of 25 were within 10% of their label. One product contained no detectable melatonin at all but did contain 31.3 mg of CBD. That's the regulatory gap in one chart — and it's why independent verification matters.

People on warfarin are often told to avoid vitamin K, then swing wildly between green-vegetable-free weeks and salad binges. NIH ODS frames it differently: vitamin K can have a serious interaction with warfarin, and if you take it, it's very important to get about the same amount of vitamin K each day from food and supplements. A sudden change is the actual danger — less can cause dangerous bleeding, more can cause blood clots.

With water-soluble vitamins, excess is largely excreted. Fat-soluble vitamins accumulate. NIH ODS reports that high intakes of preformed vitamin A can cause severe headache, blurred vision, nausea, dizziness, muscle aches and coordination problems, and in severe cases coma and death. The adult upper limit is 3,000 mcg per day from all sources, and too much in pregnancy can cause birth defects.