Unlike pills you need a doctor’s prescription for, vitamins and supplements you buy over the counter aren’t tightly checked, so what’s in them can vary a lot — sometimes they have too little of the good stuff, or even bad stuff mixed in.

Even though the government has rules for these supplements, scientists found that different brands of red yeast rice had wildly different amounts of the active ingredient—some had none at all. This proves supplements aren’t as consistent or reliable as real medicines.

This study checked 10 melatonin supplements and found that 4 of them didn’t have the amount of melatonin they claimed, and some had unknown harmful stuff in them—showing that supplements aren’t as tightly controlled as real medicines.

This study shows that dietary supplements often cause liver damage, and doctors only find out after people get sick—unlike medicines, which are tested well before sale. This suggests supplements aren’t made as carefully as real drugs.