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But the latest marketing push by drugmakers is for easy-to-use gels and patches that are aimed at a much broader population of otherwise healthy older men with low testosterone, or androgen deficiency. The condition is associated with a broad range of unpleasant symptoms ranging from insomnia to depression to erectile dysfunction. Drug companies peg this group at about 15 million American men, though federal scientists do not use such estimates.

Watson Pharmaceuticals now markets its Androderm patch, which slowly releases testosterone into the bloodstream. Abbott has its gel that can be applied to the shoulders and arms. And Eli Lilly’s Axiron is an underarm gel that rolls on like deodorant. Androderm, launched last year, had $87 million in sales, and Axiron, which was launched in 2010, had sales of $48 million last year.

“All of a sudden you’ve got these big players with a lot of money using consumer directed marketing to change the landscape,” said Dr. Natan Bar-Chama, a male reproductive specialist at Mount Sinai Hospital in New York. “They see the potential, they see the market growth annually and it’s very impressive.”

But government researchers worry that medical treatments have gotten ahead of the science.

Male testosterone is mainly produced in the testes and affects muscle mass, sperm production and various sexual characteristics. The hormone can easily be checked with a blood test, but doctors can’t agree on what constitutes a low reading in older men. Typical testosterone levels for younger men range between 300 and 1,000 nanograms per deciliter, but once levels begin dropping there is little consensus on what makes a “normal number.”

Some doctors believe testosterone levels below 300 lead to sexual dysfunction in older men, but the rule does not cover all cases. A 2010 study by researchers at the University of Manchester and other European institutions found that 25 percent of men with testosterone levels above that threshold had the same sexual problems used to diagnose low testosterone. Adding to the ambiguity is that testosterone levels change by the hour, so a man who takes a blood test for testosterone in the morning may get a completely different reading when tested in the afternoon.

SAFETY CONCERNS

Adding to the confusion over what defines “low testosterone,” there’s not much understanding of whether testosterone replacement therapy actually improves men’s symptoms. Evidence of the benefits of testosterone is mixed, and the potential health risks are serious. The largest study conducted to date, a 2008 trial involving 230 patients in the Netherlands, found no improvement in muscle strength, cognitive thinking, bone density or overall quality of life among men taking testosterone. Muscle mass increased 1.2 percent, but not enough to improve physical mobility.

The National Institute on Aging is currently conducting an 800-man trial to definitively answer whether testosterone therapy improves walking ability, sexual function, energy, memory and blood cell count in men 65 years and older. But those results aren’t expected until 2014.

In addition to concerns about testosterone’s effectiveness, the long-term side effects of the hormone are not entirely understood because most trials to date have only followed patients for a few months. But the most serious risks include heart problems and prostate cancer. In fact, all testosterone drugs carry a warning that the hormone should not be given to men who have a personal or family history of prostate cancer.

In 2010, researchers at Boston University’s school of medicine halted a large study of testosterone in senior men because patients taking the hormone were five times more likely to suffer a serious heart event, including congestive heart failure, than those taking placebos. A review of 19 testosterone trials in 2006 found that prostate cancer was significantly higher among men taking testosterone.

Also in 2006, the Endocrine Society published the first physician guidelines for prescribing testosterone for men with androgen deficiency. All six of the co-authors had received consulting fees or research funding from drug makers that market testosterone. Despite those ties, the authors took a cautious tone, stressing the difficulty of accurately diagnosing low testosterone and acknowledging that they were unable to reach an agreement about when doctors should begin therapy. They also recommend doctors have an “explicit discussion of the uncertainty about the risks and benefits of testosterone therapy.”

History has shown that hormone replacement therapy can be dangerous. That hit home for women in 2002 when a landmark study shook up the conventional wisdom about the benefits of estrogen replacement therapy for menopause, the period when women stop producing eggs and the hormone estrogen. The federal study found that women taking hormone pills were more likely to suffer heart attacks, breast cancer and strokes. Doctors now generally recommend hormones only to relieve severe menopause symptoms _ in the lowest possible dose and for the shortest possible time.

In the case of testosterone, Abbott Laboratories says Androgel and other drugs like it are an important treatment option for men with low levels of the hormone. But the company acknowledges that more study is needed.

Abbott believes that the long-term effects of testosterone replacement therapy should be studied, which is why we continually fund and support additional clinical trials, such as the National Institute of Aging’s testosterone trial,” the company says.

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