You might think that dietary supplements offer a safer, more natural alternative to medication for problems such as high cholesterol. But “some contain ingredients that are really drugs,” says Marvin M. Lipman, Consumer Reports’ chief medical adviser.

How? In some cases, naturally occurring substances are chemically identical to pharmaceuticals. In other cases, manufacturers add illegal or prescription medications to their supplements. Manufacturers aren’t required to prove that their products are safe or work as advertised before they reach the marketplace. And the Food and Drug Administration does not usually test supplements to make sure they contain only the ingredients on the label, so undeclared drugs often go undetected.

“Researchers have found prescription medications, banned pharmaceuticals and drugs that have never been tested in humans in supplements,” says dietary supplement safety expert Pieter Cohen, an assistant professor at Harvard Medical School. Those substances could interact with medications you take, cause negative side effects or lead to dangerously high doses.

Here’s what you need to know about four types of supplements that have been found to contain drugs:

  • Cholesterol-lowering
  • Sexual-enhancement
  • Weight-loss and body-building
  • Memory-enhancing