BBC Radio 5 Live has been given exclusive access to a steroid clinic in Newport, south Wales.
It's the first of its kind in the UK, where doctors and nurses carry out blood and heart-function tests on users, while also giving advice about the substances they use.
Stewart came to the clinic because he had issues with his feet. He was diagnosed with type 2 diabetes.
He was very open about the drugs he was taking: "I take two injections at least, a week, each will have 500mg testosterone and 220mg trenbolone."
He sometimes takes a third injection. Stewart's been taking these doses for two to three years.
He said he decided to start when he reached his mid-30s: "I was always tempted in the gym, it works. I'd been training naturally for a long time, and I thought, 'Hell, why not? Just try it.'"
He said they're easy to access: "A lot of the boys in the gym, as soon as they get their membership, they get their steroids as well at the same time."
Despite all the warnings about the long-term effects of steroids, Stewart is convinced that the positives outweigh the negatives.
"It's my personal choice, and I don't want to cause major illness to myself but they'll also cause a lot of benefits. Testosterone makes you feel like a teenager again in terms of sex life, gives you more confidence, so you feel more outgoing. Some people say it's aggression but it's not, it's just that you feel more confident."
The doctors at the clinic were concerned about Stewart's kidney and liver function and want to do further tests.
According to government figures, at least 300,000 people in England and Wales have taken anabolic steroids.
It's not illegal to take them, but it is illegal to buy and sell them.
NHS guidelines show that people who misuse steroids risk health problems like infertility and mood swings.
'Paul' didn't want to use his real name. He had been weightlifting since he was 15, and now, at the age 36, has just done his first 10-week course of steroids.
"Over the last eight or nine years, I've noticed I wasn't making any gains [in the gym], I couldn't change my training regime because of family or work commitments, so that's what made me think let's give it a go."
"In those 10 weeks I made gains like I'd never made before, and that surprised me. I can see from that, it's quite a hook."
Paul's wife is aware that he's taking steroids: "She's not 100%, but she said she'd rather be part of it in case something goes wrong, than me do it behind her back.
"She said she's noticed a change in me. I've become happier. And she thinks we've become a bit closer because I've opened up a bit more."
Paul had gone to the clinic to have his liver and kidney function tested, because it is weighing on his mind.
He said the only negative he's noticed is some spots: "My main worry was if I started to get 'shouty' and aggressive, especially because I'm a big guy. I didn't even get frustrated or wound up."
Dr Greg James volunteers at the Newport clinic. He outlined some of the possible side-effects of even a short course of anabolic steroids: "Bruising, rashes from injecting sites, abscess formation… Acute kidney injuries which can lead to long-term kidney problems, liver inflammations and problems. They could also have difficulties with mood swings and erectile dysfunction."
He said clients react in two very different ways if they do have serious health issues, such as kidney failure.
"We've had patients who will see that evidence presented to them, and will do an about-face and say, 'Right I'm not doing this any more, it's clearly impacting on my health,' and they stop.
"And we've got clients who will go the opposite direction and say, 'Oh well, that's reversible, this isn't going to affect my long-term health etc', which is a shame."
Joanne Hughes is a community nurse at the clinic. She said she thinks steroid use is increasing: "Among whole generations, not just the young generation. We've seen patients coming in, in their 50s and 60s who've started steroids."
Paul says he hopes if tests show steroids are having a negative impact on his health, he'd stop: "Hopefully if that ever happens I'll be clever enough to say, 'I've had my run at it.' Especially as I've got young kids. As much as I love going to the gym, I'd be confident to make the right decision."
You can hear the full report on the Emma Barnett Show on Radio 5 Live on Wednesday from 10:00 BST and then afterwards on BBC iPlayer Radio.