Many athletes and gym-goers are turning to a popular but potentially dangerous new pill to help them build muscle and gain strength: a steroid alternative known as SARMs.

The pills are widely marketed online as “legal steroids” that provide the muscle-building benefits of anabolic steroids without the troubling side effects. And while the products are legal — at least so far — their spread has alarmed health authorities, who say they are not necessarily safe.

Drug companies developed SARMs, which stands for selective androgen receptor modulators, as an alternative to anabolic steroids for people who suffer from age and disease-related muscle loss. But they are the subject of ongoing clinical trials and have not been approved for use by the Food and Drug Administration.

In October, the agency issued a public advisory cautioning that SARMs were unapproved drugs linked to “serious safety concerns,” including the potential for an increased risk of liver toxicity, heart attacks and strokes.

A month later, a study published in JAMA revealed that products marketed as SARMs were frequently misbranded and tainted with unlisted ingredients. Out of 44 products that were purchased online and analyzed, only about half contained an actual SARM, while 10 percent contained none at all. Roughly 40 percent had other hormones and unapproved drugs. Several contained a drug that was abandoned by GlaxoSmithKline a decade ago after it was found to cause cancer in animals.