Doctors at a south Wales hospital say they are alarmed at the number of bodybuilders needing treatment after the misuse of anabolic steroids.
Anabolic steroids are not illegal to use
The Royal Glamorgan in Llantrisant has recently seen as many as one case a week, the BBC has learned.
Users can develop muscle abscesses, and according to a registrar, each case costs the NHS on average £2,000.
Experts have also warned that users are becoming "sophisticated" and mixing them with other prescription drugs.
According to studies by the University of Glamorgan, steroid use is common throughout south Wales gyms.
Anabolic steroids are prescription drugs and are not illegal to use.
However, non-prescription supply of the substance is against the law.
Side effects of misuse can include liver damage and decreased fertility.
The Royal Glamorgan said it had seen a steady rise in the number of patients hospitalised because of the misuse of anabolic steroids.
Orthopaedic registrar Peter Kempshall said many patients needed overnight care.
He said: "People will go to great lengths to look good, and this is what this is - a substance that is designed to increase your muscle bulk and reduce your amount of fat and make you look good, so it is purely vanity."
Peter Kempshall says people use the drug for reasons of vanity
One anonymous 20-year-old user who was hospitalised by an abscess, said steroids were readily available.
He said: "It's easy, really easy.
"You can get it from whoever - gyms, bodybuilders, whoever wants to make money.
"It's quite cheap, you can get some quite cheap tablets - it depends what you want to do. There's different prices, depending on what you want to do."
Users are said to have become more sophisticated by mixing steroids with other drugs to force even faster muscle growth.
Professor Bruce Davies, of the University of Glamorgan, said usage was common across the south Wales valleys.
Prof Davies said: "They have amazingly sophisticated approach to this, which I suspect is usually about five years ahead of most of the endocrinologists [specialists in glands and hormones] in this country.
"They are pushing large amounts of substance into the muscle too, so all those things, I think, add up."
Andrew Penhale, aged 23, said he was sometimes alarmed by what he saw.
He goes to a gym in Trebanos, near Swansea, where steroids are banned.
"Go out to town on a Saturday night, and you see them all," he said.
"They're all really bulked up, and you can see its from steroids and it puts a lot of people off going into the city, and also playing sport because it's really unfair."