Testosterone replacement has long been prescribed for men who suffer from abnormally low levels of the male sex hormone, but overuse can lead to infertility and can even speed the growth of prostate cancer. That hasn’t stopped Michael Murray, a healthy 43-year-old home stager who works in New York and Chicago, from getting frequent testosterone injections to raise his energy level and give his bodybuilding regime a boost. “Am I making a deal with the devil? A little bit, but I have to think about my quality of life,” Murray explains. “It is like I’m in my 20s again.”

In what may become one of the most sought-after lifestyle drugs since the introduction of Pfizer’s (PFE) Viagra 14 years ago, new testosterone drugs from Eli Lilly (LLY), Abbott Laboratories (ABT), and other drugmakers are hot. Prescriptions for testosterone replacement therapies have more than doubled since 2006 to 5.6 million last year, according to data compiled by Bloomberg. Sales are expected to triple to $5 billion by 2017, forecasts Global Industry Analysts.

As many as 13.8 million men older than 45 in the U.S. have low levels of testosterone, according to a 2006 study in the International Journal of Clinical Practice. The male sex hormone begins to decline after age 30, and tends to drop about 1 percent each year. Lower-than-normal levels can lead to a loss of libido, a decrease in bone and muscle mass, and depression.

But taking the hormone holds risks. Testosterone can increase the growth of prostate tumors and cause blood clots and liver damage, says Edmund Sabanegh, chairman of urology at the Cleveland Clinic. Sabanegh has seen a rise in patients seeking a prescription for testosterone who don’t need it medically but covet its lifestyle-enhancing effects. Sabanegh also says he sees patients of other doctors taking testosterone to help with erectile dysfunction or low sex drive when they’re trying to conceive a child. Yet testosterone treatments can make men infertile, a side effect doctors sometimes fail to consider, he says. “There are a lot of really bad things that can happen” from misuse of testosterone, Sabanegh says.

Abbott spent $20.8 million on testosterone ads in 2011, according to researcher Nielsen (NLSN). One TV ad opens with a silhouette of a man on the bench as his friends play basketball. The voice-over asks viewers if they have “lost their appetite for romance?” or are “feeling like a shadow of your former self?” The ads direct viewers to a website called IsItLowT.com. On the Abbott-sponsored website is an image of a troubled-looking man sitting on the edge of a bed, his back to a woman.

Abbott subsidizes insurance co-pays of patients who use its AndroGel testosterone drug, letting users pay as little as $10 a month out of pocket. Abbott spokesman Greg Miley says the company only promotes AndroGel for Food and Drug Administration-approved uses in men diagnosed with low levels of the hormone by a doctor. “Low testosterone is a chronic but treatable disease, and our marketing efforts around disease awareness are designed to raise awareness about this,” Miley says.

Lilly began running TV, online, and print ads last year for its testosterone drug Axiron, which was approved by U.S. regulators in 2010 and is applied under the arm through a device similar to a deodorant stick. Lilly is offering a free 30-day supply of Axiron for new users. The ads are intended to “help educate men about low testosterone and encourage them to seek treatment,” says Lilly spokeswoman Teresa Shewman.

While growing, sales of the drugs aren’t on par with those of erectile dysfunction treatments. The U.S. market for testosterone replacement therapies was $1.6 billion in 2011, according to Bloomberg data. Sales of ED drugs were $5.3 billion, says IMS Health. But new testosterone prescriptions are growing fast, and facilities are opening across the U.S. to meet the demand. Johnny Mitias, an orthopedic surgeon in Mississippi, opened Ageless Men’s Health, a chain of testosterone clinics, in 2007. Mitias says he has 5,000 patients, who each pay $250 a month to receive testosterone injections. He now has 15 offices and plans to double the number next year.

The bottom line: Sales of testosterone drugs are booming. Although 13.8 million men suffer from low testosterone, others covet the hormone’s libido lift.