Over the past few years anyone who's turned on a TV or opened up a Web browser has probably been bombarded with come-ons about low testosterone – or "low T" – and the crucial role the hormone plays in a man's health. And with good reason. After all, the problem of testosterone deficiency is in many ways the perfect medical problem: It's easy to correct with supplemental doses of the hormone, and treating it combats an array of inevitable, age-related conditions: depression, low energy, inability to build muscle, trouble sleeping, waning libido, even heart problems. Now, thanks to a savvy, shame-free marketing campaign by various companies offering testosterone gels – the "low T" business is booming. Sales of supplemental testosterone have more than doubled since 2006 and there are as many as 5.6 million men estimated to be currently taking testosterone. Men over 40 are showing up at their doctor's office wondering if their T levels are high enough, and whether they could benefit from cranking them up a notch.

But, as promising as hormone replacement is, the intricacies of our body chemistry largely remain a mystery, and there's still little consensus over which patients actually need testosterone supplements and what levels are considered abnormally low. Though the potential repercussions of the therapy – the lowering of sperm production and the possible hastening of existing prostate cancer – are understood and acknowledged, there is no shortage of men looking to take advantage of a little hormonal edge.

"You have all the drug companies pushing their formulations, and saying that testosterone replacement is the be-all, end-all," says Dr. Gregory Bernstein, a urologist at Washington Urology at Virginia Hospital Center. "But the fact is, this is sort of in its infancy still. There's a lot we don't know about testosterone."

Scott Berliner, a clinical integrative pharmacist who has been administering hormone therapy, including testosterone, for more than 25 years, believes that we need to look at the causes of low testosterone rather than merely replacing what the body is lacking. Two culprits, he says, are stress and the abundance of toxins – from the parabens and heavy metals in our deodorant and shampoos to the chemicals in our food – that raise male estrogen levels while lowering testosterone. Berliner points to a fairly complicated process called "cortisol steal" in which cortisol, a sort of superhormone the body uses to cope with stress and to process information, can throw off testosterone production by robbing the body of other hormones (namely DHEA) it would otherwise use to make testosterone. "Basically when you have a lot of stress and a lot of information to process, the body's requirement of cortisol goes up, and often the body will rob [from the DHEA] to make more cortisol rather than testosterone. That's why stress can lower libido. Combine that with the amount of toxins the average person consumes, and the process of making testosterone is disrupted." Before beginning a program of testosterone, Berliner suggests patients first limit their intake of testosterone-inhibiting chemicals by switching to chemical-free grooming products and adopting an organic diet.

Some physicians are beginning to take a more measured approach because the therapy is still relatively new and may have regrettable side effects. Exacerbating any existing prostate cancer is the main worry, and testosterone therapy is not recommended for men who have the disease or have a high risk of the disease. Bernstein exercises greater vigilance of the prostate in patients who are on testosterone therapy. "There are no studies that show that testosterone replacement causes prostate cancer," he says, "but if you're going to go on testosterone replacement, you need to be extra vigilant in your prostate-cancer screening now that we've revved up the system. These prostate cells are getting stimulated by this extra testosterone, and now you've sort of unleashed the cancer cells to start multiplying as well."

Another, more surprising, potential effect of supplemental testosterone is a drop in sperm production. So when treating patients who are suffering from low T who want to remain fertile, Bernstein prescribes clomiphene citrate (Clomid is one brand name), which stimulates sperm production in the testes as it also increases testosterone. "It's a way to stimulate the body to make its own increase in testosterone rather than taking it externally," he says. Similarly, other physicians prescribe not testosterone replacement, but a supplement of the hormonal precursors to testosterone, like the steroid DHEA.

Beyond decreased sperm production, another repercussion you don't see in the pop-up window ads asking "Is it low T?" is dependence: Once you begin taking testosterone, it's very difficult to stop because the body accommodates for the extra hormone and slows its natural production of it. In reality, replacing hormones is a lifelong commitment. Perhaps that's why some doctors are counseling patients to use caution and to look for other solutions before signing on for full testosterone therapy. Others see the gradual decrease in testosterone as a natural form of aging that we can combat by building muscle and reducing stress, even while we accept it and learn to live with it.

"I think too many people come in to see me thinking that giving them testosterone is going to be the fix for all their problems," says Bernstein, "and that's not the case. It needs to be used in the right scenario."

Dr. Ronald Swerdloff, Chief of Endocrinology at Harbor-UCLA Medical Center and one of the authors of the Endocrine Society's recent guidelines, which advocated prescribing the therapy only to men who both exhibit symptoms and show low levels, sees both sides. "There are undoubtedly people who are being treated that don't meet the best guidelines. But there are many people who do meet the guidelines who aren't being treated."

Related links:

Is DHEA a Better Way to Raise Testosterone?

What's Normal Testosterone, Anyway?