A supplement marketed at middle-aged men as a way to boost testosterone levels may not work - and could even cause harm.
Even the existence of a "male menopause" is debated
Officials at the UK's Medicines and Healthcare Products Regulatory Agency say that it cannot be sold legally in the UK - but it is simple to find the supplement on sale on the internet.
Androstenedione was tested by experts at the University of Texas at Arlington, who gave it to volunteers for four weeks.
They found that, at the end of that period, there was no discernable change in the hormone testosterone itself - and no changes in body fat levels, resting heart rate, blood pressure, even though advertising for the product occasionally claims that it can benefit these.
Doctors, however, are concerned that long-term supplementation raises the risk of shifting the delicate balance of the body's hormones and causing as-yet unknown health problems.
The theory behind the supplement is that men experience symptoms in middle age which may be due to a drop in the level of the hormone testosterone.
The existence of this "male menopause" is by no means certain, although many men report symptoms such as fatigue, irritability, weight gain and loss of libido have all been attributed to a hormonal shift during this period.
Some doctors recommend injections of the testosterone to reverse these effects.
However, androstenedione is a substance that is a "precursor" to testosterone - meaning the body can turn it into testosterone.
This is why the supplement has been marketed as a possible treatment for symptoms of the "andropause".
The latest study, published in the British Journal of Sports Medicine, adds to other evidence suggesting that this supplement, and others like it, may have little benefit, say its authors.
A spokesman for the Medicines and Healthcare Products Regulatory Agency (MHRA), formerly the Medicines Control Agency, confirmed that products containing androstenedione could not be legally sold in the UK.
This is because the agency has ruled that the chemical should be regarded as a medicine rather than a health supplement, meaning it requires a licence from the MHRA.
No such restrictions have been placed on the supplement in the US, where it is favoured by bodybuilders as well as marketed at middle-aged men.