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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

Red yeast rice supplements are touted as a way to reduce “bad” LDL cholesterol. They may do so, because some contain substantial amounts of monacolin K, a chemical identical to the prescription drug 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

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 drugs for erectile dysfunction 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 and nitroglycerin.

Weight-loss and bodybuilding

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 sometimes contain a speedlike substance called beta-methylphenethylamine. And the stimulant drug methylsynephrine (also called oxilofrine) has been found on labels.

Another hint: “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 can also interact dangerously with some prescription meds.

Memory-enhancing

The labels of certain brain-boosting supplements list drugs such as vinpocetine, which is used in China, Germany and Russia to treat stroke and cognitive impairment. It’s not an approved medication in the United States, but because of a quirk in the law, it is permitted in dietary supplements. (The FDA is assessing the substance, so that may change.)

Vinpocetine is in roughly 375 supplements sold in the United States, according to Natural Medicines, a database compiled by the Therapeutic Research Center. A 2015 study, led by Harvard’s Cohen and published in the journal Drug Testing and Analysis, found that many products contained prescription doses. Cohen’s team also found picamilon — an ingredient that is not approved in the United States but is used to treat neurological disorders in Russia — in supplements.

Vinpocetine may cause headaches, flushing and low blood pressure, and it may interact with nitrates, which are used to treat angina (chest pain), and with blood thinners, such as coumadin (Warfarin and generic). With picamilon, “the side effects and potential interactions are largely unknown,” Lipman says.

Consumer Reports’ advice: Skip supplements and see your doctor. “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.”

Copyright 2016. Consumers Union of United States Inc.

For further guidance, go to www.ConsumerReports.org/Health, where more detailed information, including CR’s ratings of prescription drugs, treatments, hospitals and healthy-living products, is available to subscribers.