Some bodybuilders abuse the cancer drug tamoxifen to negate the breast-boosting effects of steroids, University of Glamorgan researchers found.
Bodybuilders abuse steroids to build muscles
Steroids are derived from testosterone, but if levels of the male hormone are too high in the body, it is converted into oestrogen and men grow breasts.
A survey of 200 gym users found 22% took tamoxifen to reverse this effect.
Cancer experts expressed concern over the findings, published in the Journal of the Royal Society of Medicine.
The researchers compared the results of this survey, carried out last year, with one carried out in 2000 when just one to two per cent said they had taken tamoxifen.
They also found 11% of bodybuilders took human chorionic gonadotrophin to stimulate the testes, which wither due to excessive steroid use.
The researchers, from the University of Glamorgan's Health and Exercise Science Research Unit, say bodybuilders turn to steroids because they see themselves as too "weedy" - in a condition which is the opposite of anorexia.
But they are then forced to move on to using a cocktail of other drugs to counteract the effects of the steroids.
Professor Bruce Davies, who led the research, said: "Gym-goers are increasingly resorting to steroids and prescription drugs to counteract their altered body images.
"Sufferers of negative or heightened body image issues will go to extremes to meet their desired goals and doctors need to be more aware of the efforts 'bigorexics' will go to."
"Our results corroborate research undertaken in the US, that is, an increasing number of non-athletes are resorting to a combination of drugs with the sole intention of improving their physical appearance."
Professor Davies also warned bodybuilders were risking abscesses in their muscles, because they were injecting counterfeit drugs using aftershave rather than alcohol as a "sterilising" agent.
He called for the medical profession to recognise the needs of this group.
"There needs to be greater awareness of the psychological reasons of prescription drug and steroid abuse, and counselling and appropriate medical treatment made available."
Dr Kat Arney, cancer information officer at Cancer Research UK, said: "Tamoxifen is a potent and effective drug for treating breast cancer. Like all medication it can have side effects, so it should not be used without medical supervision.
"Bodybuilders who take tamoxifen or other inappropriate prescription drugs are gambling with their health."