The number of underground 'steroid labs' in Wales which sell illegal performance-enhancing drugs has risen and is a cause for concern, says Public Health Wales (PHW).
The situation is getting worse because of a new law making it harder to import steroids, health officials have said.
Pharmacists and drugs workers have also reported seeing more steroid users accessing needle exchange programmes.
Injecting steroids can cause ulcers and gynecomastia - where men grow breasts.
It also increases the risk of developing hepatitis or HIV.
Until last year, users could buy steroids online from outside the UK and get them delivered.
But the law changed meaning users have to bring them into the country personally.
Amateur sportsman Peter (not his real name) said the steroid user is easy to spot in the gym.
"You might see someone coming to the gym and they're not eating properly and not training properly yet the next time you see them they are about 100% stronger than they were a week or two before," he said.
He told BBC Radio Wales' Good Morning Wales programme that illegal steroids could be found by word-of-mouth.
"The odd person produces it themselves. Say they've done a bio-chemical degree or something like that... they might have their own little lab set up," he explained.
"They may have something which is five times stronger than the regular stuff. They can sort of tailor-make it.
"Someone takes your order, the person knocks them up, and they get sold on."
Josie Smith, the national lead for substance misuse with Public Health Wales said: "Because of the change in the law our concern is there is an increase in the manufacture of steroids and image-enhancing drugs in underground labs," she said.
"These are not regulated; the substances they are manufacturing are not regulated.
"My understanding is that some are quite well-run laboratories. However I am aware also that perhaps some less conscientious individuals may be trying to produce substances in garages."
It is legal to possess steroids for personal use as long they have not been manufactured on the black market but it is illegal to make or supply steroids without a licence.
Bruce Davies, professor of applied physiology at University of South Wales, said one of the main problems with people taking anabolic steroids is non-sterile injection.
"If on top of that you super-impose a situation where the materials they're injecting could also be non-sterile I think you've already exacerbated an already serious problem," he added.
The Serious Organised Crime Agency (SOCA) says the steroid trade is closely linked to the supply of class A drugs.
The mark-up in profit on steroids can be around 400% according to the agency.
Professor Martin Davies, a pharmacist working in Newport, said steroid users were increasingly accessing needle exchange programmes, originally aimed at heroin users.
"Going back two to three years the number of (steroid using) clients we were seeing was about three to four percent of those who came in," he said.
"That has now increased to 30-40%."
Experts warn the next generation of performance enhancing drugs is already emerging - a mixture of bacteria and genes injected into muscles.
Prof Davies said: "The two things I'm always amazed about are the pharmacological knowledge that these people have and how quickly the drugs are made available to body builders.
"We're talking weeks, weeks, and they're using these things."