Some supplements that claim to boost athletic performance could cause heart attack or liver damage, a researcher says

• Herbal supplements are often marked as being a safe and "natural" way to improve athletic performance.
• But herbal supplements can pose risks, including harm to the liver and heart.

Taking a "natural" supplement might feel safe, but even herbal supplements may carry risks, one researcher says. Athletes who take herbal supplements to increase athletic performance and build muscle mass should be particularly wary of potential harms from supplements.

"Weight loss and bodybuilding supplements are probably the most problematic" for their potential to cause liver damage, Dr. Bill Gurley, principal scientist in the National Center for Natural Products Research, told Insider.

In addition to liver damage, weight loss and bodybuilding supplements can also cause injuries to your heart and blood vessels, he said.

While "the vast majority of dietary supplements are quite safe," Gurley said, the risk of dangerous side effects increases when a product contains many different herbal compounds, or herbal supplements are used in conjunction with prescribed medication.

Here are the supplements that Gurley said athletes should avoid because they can be particularly dangerous.

Supplements with added caffeine

Supplements touted for athletic performance are often loaded with added caffeine to boost energy, but this can be problematic.

While "caffeine, for the most part, is pretty innocuous," Gurley said, in athletic supplements "a lot of times, you don't know how much caffeine you're taking."

More than one gram of caffeine can create a "toxic manifestation," Gurley said. Especially if the product contains "other compounds that act similarly to caffeine in terms of their cardiovascular stimulant properties, then that just exacerbates the problem," he said.

"Exercise alone puts a significant strain on your cardiovascular system," Gurley said, "throw in a bunch of stimulants that are known to have adverse cardiovascular effects, and it's just a bad combination."

For example, too much caffeine can make it harder to tell if you're becoming overheated.

Caffeine can also be a problem if you're taking prescription blood pressure medication, because it can raise your blood pressure.

Natural steroids

Be wary of products containing "natural anabolic steroids," said Gurley. Natural steroids are ingredients derived from herbs, plants and other sources that are thought to emulate the function of human hormones and steroids, according to Medical News Today.

But oftentimes, products labeled as having "natural anabolic steroids" actually contain synthetic anabolic steroids. That's because there's largely no federal regulation in the supplement industry — companies can produce and sell supplements without notifying any government agency.

And although synthetic steroids might help "bulk up your muscle mass," Gurley said, "you also wreak havoc on your liver the longer you take these things."

Black pepper extract

On its own, black pepper extract is innocuous. But it can be risky when combined with other compounds, including other herbal supplements or prescription medication.

"You'll find black pepper extract is oftentimes a fairly common ingredient in a lot of multicomponent supplements," said Gurley. The reason, Gurley said, is because black pepper extract makes it easier for compounds to enter your bloodstream, and for you to feel an effect.

But this can also be dangerous, as it "can increase the risk of an herb-drug interaction," Gurley said.

Yohimbe

Derived from bark of the Yohimbe tree, which is native to central and western Africa, Yohimbe is touted for weight loss, high blood pressure, erectile dysfunction and athletic performance.

But it can also be dangerous. Yohimbe contains a chemical called yohimbine which can cause a heart attack or irregular heartbeat, according to WebMD. It has also been linked to seizures, high blood pressure, and anxiety.

"I always recommend you avoid Yohimbe, as much as you can," Gurley said.

Supplements with more than one ingredient

Supplements marketed for athletic performance, including herbal supplements, often contain many different compounds. "When you start getting into these crazy combinations that are in these bodybuilding supplements, that's when things can get a little bizarre," Gurley said.

Gurley said taking these multi-ingredient supplements is like opening a "pharmacological Pandora's box," and there's a greater risk that a dangerous drug interaction could occur.

People should be especially careful taking herbal supplements if they are taking prescription medication, Gurley said.

Taking prescription medications and herbal supplements at the same time can sometimes lead to unintended, and potentially harmful, herb-drug interactions. So be sure to check with your doctor before trying any new supplements.

Be mindful of where you buy your herbal supplements

For all herbal supplements, Gurley said it's important to pay attention to who you're buying from. Some supplement makers aren't as careful manufacturing their supplements, which can lead to contamination and adulteration — where compounds that aren't advertised are snuck into the supplement.

"You'll be shocked at how many weight loss and dietary and bodybuilding supplements are adulterated with conventional medications," Gurley said. These adulterated products can increase the risk of liver injury.

To avoid this, Gurley recommends buying herbal products that are HSP or NSF sport certified.