Anabolic steroids, abused by athletes to increase performance, weaken the body's natural defence against infections and cancer, tests show.
Some athletes abuse steroids to enhance their performance
Australian scientists found that, even at doses 50 times lower than those commonly used by abusers, the drugs dampened the immune system.
Volunteers testing the drugs for Southern Cross University also reported changes in their personality.
The reduced empathy they felt may explain the condition 'roid rage'.
This is when people taking steroids become overly aggressive.
For six weeks, a group of 24 athletes from different countries agreed to take anabolic steroids for the purpose of the investigation.
At the beginning of the study the volunteers took part in a series of athletic events to measure their personal as well as their competitive performances, and their strength, speed and endurance.
They were divided into two groups, one group receiving injections of the anabolic steroid testosterone enanthate (3.5 milligrams per kilogram of bodyweight per week) and the other receiving harmless dummy injections.
Neither group nor the scientists overseeing the experiment, commissioned by New Scientist and Channel 4, knew who was given what.
All the volunteers were then put through a training regime under the supervision of an Olympic standard coach.
As expected, the steroids increased the athletes' performance, particularly by increasing the body's strength and power, but not without side effects.
The effectiveness of a type of white blood cell crucial to the body's immune system, Natural Killer (NK) cells, was reduced by 20% among those in the steroid group.
Lead researcher Dr Robert Weatherby said: "For the first time we've been able to show that androgenic anabolic steroids have a seriously detrimental effect on a specific part of the human body's immune system.
"Anyone taking steroids over a long period of time is potentially seriously endangering their health in the long term."
The athletes who had taken steroids also reported a change in their personality.
The steroids appeared to reduce their empathy for other people, making them less sensitive to the effect of their actions on others.
This finding is significant because very few drugs cause an actual change to the personality rather than just heightening mood swings, after which the user returns to normal, say the researchers.
The changes to personality would result in a less empathetic, more compliant person who would take less responsibility for their actions, they said.
This might have legal implications because people using steroids who are charged with a criminal offence may be able to argue a defence of diminished responsibility, they added.
John Brewer, head of sports science at Lilleshall Sports Injury and Human Performance Centre in Shropshire, said: "That's not surprising, nor is it just about the immune system.
"It is likely you will have problems with your heart and potentially with organs like the liver and kidneys.
"Some of the side effects are not necessarily seen for years to come.
"The message is a very strong, loud and clear 'Do not take steroids.'
"You might have short term gains in terms of performance but the immediate and subsequent long term damage to health is immense and potentially fatal," he said.