Steroid use may have a lasting effect on teenagers' brains, a study suggests.
Some body-builders use steroids to help them gain muscle
US researchers, who looked at the effect of anabolic steroids on hamsters, suggest the drugs 'flip a switch' and trigger lasting aggression.
The effects of steroid may last for at least two years, and cause permanent brain changes, the Behavioral Neuroscience study warns.
But a UK expert said it was impossible to state the length of effect in humans, based on a study in hamsters.
Long-term steroid users can suffer from mood swings, hallucinations and paranoia, liver damage and high blood pressure as well as increased risk of heart disease, stroke and some types of cancer.
Coming off steroids can lead to depression.
In the study, by a team at Northeastern University, Boston, researchers studied the behaviour of adolescent hamsters when another hamster was put into their cage.
Hamsters naturally defend their territory - even if they are normally mild-mannered - by play-fighting, wrestling and nibbling, the researchers found.
But hamsters injected with commonly used steroids, which were suspended in oil, became extremely aggressive.
Even after the drug was withdrawn, the newly vicious hamsters attacked, bit and chased the intruders.
Their aggressiveness was 10 times greater than that of other hamsters which were only injected with oil.
The effects lasted for almost two weeks, the equivalent of half their adolescence.
After this period, the animals reverted to their normal playful defensiveness.
Post-mortems on the hamsters found there had been also been changes in their brain activity.
While they were being given steroids, a part of their brains called the anterior hypothalamus, which regulates aggression and social behaviour, pumped out more of a neurotransmitter called vasopressin.
Three weeks after withdrawal, vasopressin levels had subsided in line with the aggressive behaviour.
Dr Richard Melloni, who led the research, said that because the part of the brain which his team studied is similar in rodents and humans, the scientists say their findings are probably applicable to people.
Dr Melloni added: "Because the developing brain is more adaptable and pliable, steroids could change the trajectory if administered during development."
"If you hit the right areas of the brain at the right time, you make permanent changes."
And he said: "It's our hope that people considering the use of these drugs weigh the long-term health risks and the serious potential for aggression and violence."
But the researchers said the findings could potentially aid the development of treatments for aggressive behaviour, whether or not it has been caused by steroid abuse.
However, Professor Jonathan Seckl of the University of Edinburgh, said: "Steroids are linked to aggressive behaviour and that can persist afterwards.
"But this study showed effects for a few days, and you can't say that will translate into years for humans."
He said the effects seen in the hamsters could simply have been down to steroids' slow-release mechanism - which means their effects could still be seen in the body for some days after stopping taking the drugs.
But he said: "This study does show you get really significant behavioural effects from steroids - in animals and humans.
"I see plenty of people who abuse steroids, usually young men into athletics or weight-lifting, and the effects do go on for a long time after they have stopped."