Sales of prescription testosterone gels that are absorbed through the skin generated over US$2 billion in US sales last year, a number that is expected to more than double by 2017. Abbott Laboratories, which owned AbbVie until January 1, spent US$80 million advertising its version, AndroGel, last year.

Once a niche treatment for people suffering from hormonal deficiencies caused by such medical problems as endocrine tumours or the disruptive effects of chemotherapy, the prescription gels are increasingly being sold as lifestyle products, to raise dipping levels of the male sex hormone as men age.

"The market for testosterone gels evolved because there is an appetite among men and because there is advertising," said Dr Joel Finkelstein, an associate professor at Harvard Medical School who is studying male hormone changes with ageing.

"The problem is that no one has proved that it works and we don't know the risks."

Dr Eric Topol, a cardiologist at Scripps Health in San Diego, is alarmed by the high percentage of patients he sees who use the roll-on prescription products, achieving testosterone levels that he described as "ridiculously high".

The gels are of questionable medical benefit for many of the millions of men who now take them, he and other doctors say, and their side effects may well prove dangerous.

"These medicines come with a risk of coronary artery disease," Topol said.

"When I ask patients why they're on it, the instant response, is, 'I have low T.' I ask, 'Why would you even get tested for that?'" he said.

Nevertheless, many insurers cover most of the cost of the high-priced hormone treatments. AndroGel and another popular testosterone gel, Axiron, by Eli Lilly & Co, sell for more than US$500 a month retail.

While television viewers are barraged with advertising warning men that they may have "low T", Finkelstein said, "There is no such disease."