Too much testosterone can kill brain cells, say United States researchers.
Their finding may help to explain why steroid abuse can cause behaviour changes such as aggressiveness and suicidal tendencies.
Tests on brain cells in laboratory dishes showed that while a little of the male hormone is good, too much of it causes cells to self-destruct in a process similar to that seen in brain illnesses such as Alzheimer's.
"Too little testosterone is bad, too much is bad but the right amount is perfect," said Barbara Ehrlich of Yale University in Connecticut, who led the study.
Testosterone is key to the development, differentiation and growth of cells and is produced by both men and women, although men produce about 20 times more of the hormone.
It can also be abused, and recent scandals have involved athletes who use the hormone, or steroids that turn into testosterone in the body, for an unfair advantage.
"Other people have shown that high levels of steroid can cause behavioural changes," said Professor Ehrlich.
"We can show that when you have high levels of steroids, you have high testosterone and that can destroy the nerve cells. We know that when you lose brain cells you lose function."
Ehrlich's team tried the same thing with the "female" hormone oestrogen, just to be fair.
"We were surprised, but it actually looks like oestrogen is neuro-protective. If anything, there is less cell death in the presence of oestrogen," she said.
Writing in the Journal of Biological Chemistry, Professor Ehrlich and team said their findings meant people should think twice about supplementing with testosterone, even if it does build muscle mass and aid recovery after exercise.
"These effects of testosterone on neurons will have long-term effects on brain function," they wrote.
"Next time a muscle-bound guy in a sports car cuts you off on the highway, don't get mad - just take a deep breath and realise that it might not be his fault," said Professor Ehrlich.
The cells die via a process called apoptosis, also known as cell suicide or programmed cell death.
"Apoptosis is an important thing for the brain - the brain needs to weed out some of the cells. But when it happens too frequently, you lose too many cells and causes problems."
A similar process is seen in Alzheimer's disease, and Huntington's disease, another fatal brain illness.
"Our results suggest that the responses to elevated testosterone can be compared with these pathophysiological conditions."
