Older men with low testosterone levels might benefit from hormone treatment for healthy bones, but it may also increase their risk of heart problems, US trials suggest.
Levels of the male hormone gradually decrease naturally as men get older.
Doctors agree men of any age with very low levels and symptoms such as low libido and erectile dysfunction should be considered for testosterone therapy.
But the debate rages about where the treatment cut-off level should sit.
In the US, selling testosterone direct to consumers has become big business.
In the UK, men have to see a doctor to get a prescription, although some buy it on the internet.
All of the men in the US trials were 65 or older and had moderately low testosterone levels, but with no obvious medical cause.
Half were given testosterone gel, applied to the skin, while the other half were given a placebo gel containing no testosterone.
The 700 men, treated at different research centres, were monitored for a year and the findings are published in the JAMA and JAMA Internal Medicine.
At 12 months, testosterone treatment was linked to:
The researchers say more studies are needed to check if this means that men who take testosterone are then at increased risk of heart problems.
Another study published in JAMA Internal Medicine that tracked heart health over a period of about three years suggests possibly not, but experts say it is too soon to be sure.
Other research has suggested testosterone might help protect the heart, but experts say it is too soon to be sure.
There is also a concern that testosterone treatment might make prostate cancer more likely, by fuelling tumour growth.
Dr Channa Jayasena, an expert at Imperial College London and a spokesman for the UK Society for Endocrinology, said: "The message here is to select the correct patients who will benefit from testosterone therapy, but not treat it as a wonder-drug."
The European association of Urology recommends other possible causes of low testosterone, such as obesity, should be treated before testosterone therapy is considered.