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

“Researchers have found prescription medications, banned pharmaceuticals, and drugs that have never been tested in humans in supplements,” adds dietary supplement safety expert Pieter Cohen, M.D., an assistant professor at Harvard Medical School.

Those substances, says Cohen, could interact with medications you take, cause negative side effects, or lead to dangerously high doses.

And there's often no way to know for sure what a supplement contains from reading its label. That's because the Food and Drug Administration does not usually test supplements to make sure they contain only the ingredients on the label.

Weight loss supplements are a common place for medications to hide. But they're not the only supplements to be wary of.

Here's the rundown on drugs that have been found in weight loss supplements and several other kinds of products.  

Bodybuilding and Weight Loss Supplements

In 2015 alone, at least 53 weight-loss supplements were found to contain actual drugs, including two that the FDA pulled from the market because of health risks: the prescription weight-loss drug sibutramine and the laxative phenolphthalein.

There’s often no way to know for sure what a product contains from reading the label, but you might spot some clues.

For example, one study showed that supplements listing the ingredient acacia rigidula may contain a speed-like substance called beta-­methylphenethylamine.

Sometimes, medications are listed right on supplement labels. For example, the stimulant drug methylsynephrine (also called oxilofrine) has been found on ­labels for weight loss and bodybuilding supplements.

DMAA” and “geranium extract” on ingredients lists are code for dimethylamylamine, a substance suspected in the deaths of two people in 2011.

Those drugs can cause a host of side effects, from increased blood pressure to cardiovascular problems and heart attacks.

They may also interact dangerously with prescription medications for conditions such as ADHD and high blood pressure.

Cholesterol Lowering Supplements

Red yeast rice supplements are touted as a way to reduce “bad” LDL cholesterol.

Those supplements might have cholesterol-lowering effects, because some contain substantial amounts of monacolin K, a chemical identical to the prescription statin lovastatin (Mevacor and generic).

But just like statins, the products can cause muscle, liver, and kidney problems.

“And if you’re already taking statins, adding red yeast rice increases your risk of those side effects,” Lipman says. “But unlike statins that contain known doses, the amount of monacolin K in supplements can vary greatly from pill to pill.”

Sexual Enhancement Supplements

A study of 150 sex supplements, published in the Journal of Pharmaceutical and Biomedical Analysis in 2015, found that 61 percent were adulterated with drugs.

Some of the ingredients were unapproved and untested, but prescription erectile dysfunction meds sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra) were also frequently found.

Supplements that contain prescription erectile dysfunction drugs can cause dizziness, headache, flushing, stomach upset, and blurred vision.

They can also lead to dangerously low blood pressure and death when combined with heart medications such as nitrates or nitroglycerin.

Memory Enhancing Supplements

Certain brain-boosting supplement labels list drugs such as vinpocetine, which is used to treat stroke and cognitive impairment in China, Germany, and Russia.

It’s not an approved medication in the U.S., but because of a quirk in the law, it is permitted in dietary supplements. (The FDA is taking a closer look at vinpocetine, so that might change in the future.)

Vinpocetine is in roughly 375 supplements sold in the U.S., according to Natural Medicines, a database compiled by the Therapeutic Research Center. A 2015 study, led by Cohen and published in the journal Drug Testing and Analysis, found that many products contained prescription-level doses.

Cohen’s team also found picamilon—an ingredient that is unapproved in the U.S. but used to treat neurological disorders in Russia—in some supplements.

Vinpocetine may cause side effects such as headaches, flushing, and low blood pressure. It may interact with nitrates, which are used to treat angina (chest pain), and blood thinners such as coumadin (Warfarin and generic).

When it comes to picamilon, “the side effects and potential interactions are largely unknown,” Lipman says.

Protect Yourself

So, what should you do? Skip supplements and see your doctor for health concerns.

“In many cases, lifestyle improvements may be the best medicine,” Lipman says. “And if treatment is needed, opt for regulated medications with known risks and side effects that can be monitored carefully.”