Many men use a testosterone supplement to treat symptoms linked to testosterone deficiency in older age, such as poor libido and deteriorating muscle strength. However, such treatment appears to come with additional health risks for frail, elderly men who use the gel to increase their strength and mobility.

Researchers reported Wednesday that a study designed to assess the risks and benefits of testosterone replacement therapy in men ages 65 and older who had mobility problems was stopped early -- in December 2009 -- because of a much higher rate of cardiovascular problems in the men receiving testosterone compared with those receiving a placebo gel.

The study, led by researchers at Boston University and funded by the National Institute on Aging, included 209 men, half of whom used testosterone gel daily for six months. The average age of the men in the study was 74. All of the men in the study had limited mobility and a high prevalence of chronic disease. However, among the 106 men using testosterone gel, 23 had cardiovascular-related events compared with only five men in the 103-person placebo gel group. The study is published online in the New England Journal of Medicine.

It's difficult to know just what to make of this study, the authors acknowledged. There was a broad range of cardiovascular events, including heart attack, stroke, death from suspected heart attack, exacerbation of congestive heart failure, atrial fibrillation, swelling, chest pain and other milder symptoms. With such a diversity of events, it's hard to pinpoint how testosterone replacement therapy may be responsible. The doses of testosterone used in the trial may have been higher than in previous testosterone studies that have not found cardiovascular risks. Moreover, the trial was small and was stopped early.

The elderly men using testosterone did show increased strength in leg-press and chest-press strength and in stair climbing while carrying a load.

As for younger and healthier men who use testosterone gel, it's doubtful this study can be applied to them, the authors said. Previous studies of testosterone gel in other groups of men have not shown significant increases in cardiovascular risks. "Caution is also warranted in extrapolating these findings to other doses and formulations of testosterone or to other populations, particularly young men who have hypogonadism without cardiovascular disease or limitations in mobility," they wrote.

However, long-term studies on the risks and benefits of testosterone therapy in men of all ages are lacking, according to a recent story in the Los Angeles Times. Meanwhile, the availability of testosterone in gel form has led to a huge jump in prescriptions in the U.S.; 3.3 million prescriptions were filled in 2008 compared to 64,800 in 1999, according to one pharmaceutical data company.

As for older men in poorer health: "The authors note that physicians and patients, especially older men, should consider this study's findings on adverse effects along with other information on the risks and benefits of testosterone therapy," officials from the NIA said in a statement. Further research is needed to clarify the safety issues raised by the trial, the authors note.

Additional trials on the risks and benefits of testosterone therapy in men are ongoing, the NIA statement said. Those trials will continue although investigators and participants will be advised of the findings of the Boston University study.