Harry (not his real name), 27, a marketing executive from north London, is a keen sportsman and bodybuilder. He spent hours in the gym, and poring over health pages for muscle-boosting tips. Yet he grew frustrated when his muscle growth appeared to plateau. While many bodybuilders turn to steroids (some 250,000 people are thought to use them in the UK, as Raoul Moat apparently did), Harry was deterred by the side-effects, which can include mental health damage. Instead, like an increasing number of gym users, he turned to Kigtropin.

A brand name for synthetically produced human growth hormone, Kigtropin is used to replace the naturally produced hormones in the pituitary gland, which slow down as we leave our teens. It was once an expensive niche drug costing thousands of pounds a dose, but is now becoming more common in high street gyms across the UK. In 2007, Sylvester Stallone was ordered to pay £5,400 in fines and costs by a court in Australia for possession of growth hormone. This year, Tiger Woods's former doctor Anthony Galea was charged with possession of growth hormone and administering it to clients.

Now, thanks to cheap supplies available on the internet (mainly from China), Kigtropin has hit the mainstream. In Bristol, bosses at a branch of Fitness First had to install needle bins earlier this year because so many members were leaving syringes lying around. A spokesman for Fitness First said the gym did not tolerate the use of drugs and was "increasing monitoring procedures to identify any unacceptable or illegal behaviour".

But for Harry, the drug seemed the perfect solution."I have always wanted to be much bigger. I went to a sport-playing school and always felt smaller than the other guys. What I had heard about growth hormone was unbelievable. Being in a gym where people take it, you assume everyone is at it."

He began taking the hormone for 18 months in cycles – three months on it, one month off – and was thrilled by the results. "I can lift more, my muscles feel harder, I have increased energy and I don't have the paranoia or 'roid rage [the anger brought on by steroid abuse] I might have had with steroids. I tore my achilles tendon playing rugby last year. The doctor said I would be out for nine months, but my tendon healed within three and I was back playing within four months. I think that had a lot to do with what I was taking."

Dr Michael Graham, senior lecturer in substance misuse at Newman University College, Birmingham, says: "Growth hormone has extremely therapeutic benefits. It is prescribed privately by Harley Street clinicians who assist in anti-ageing. But it also can enhance muscle growth and promote weight loss by preventing carbohydrate from being turned into fat.

"I have carried out a study which showed that human growth hormone increased muscle mass in steroid users whose muscle growth had flattened out. Also, it has been shown to increase cartilage growth and repair – there is no shadow of a doubt that users will have an increased healing rate."

Yet doctors warn that growth hormones are illegal without a licence – those found supplying them can face 14 years in prison and an unlimited fine. Even more worryingly, users of the hormone could be dicing with death. Nearly all of the Kigtropin entering this country is smuggled in or bought online with no control or guidance on how to take it. Mick Hart, author of the Layman's Guide to Steroids, says: "The danger is 99% will use it irresponsibly – taking way too much or not knowing how to inject it. Dealers want you to take as much as they can sell you. Cycles of hormone use used to be around eight weeks long and then some time off – now people are taking them solidly for two to three years."

Inexperienced syringe users can slash an artery and bleed to death, create blood clots, or hit a nerve and risk permanent paralysis. Long-term use can, according to Graham, lead to carpal tunnel syndrome (the compression of nerves in the wrists, which causes incessant tingling), raised blood sugar levels (which can trigger Type 2 diabetes), heart failure and – in excessive doses – gigantism, the disproportionate growth of body parts.

Users also have no guarantee of what they are buying, according to drug seizure expert Allen Morgan. "I have had cases where dealers didn't even know that they had been selling rubbish. From a law enforcement perspective it is a grey area, as police are brought up on a culture of going after street drugs and they simply have no grasp of how the bodybuilding drugs market works."

Hart says that supplies could also be tainted: "They are finding trace elements of metals in phials being shipped in from all over the place, as any wannabe dealer with a metal drum in places like China and Russia is attempting to make them on the cheap. That can be lethal."

However, for Harry and many others the lure of the physique of their dreams is too strong to give up: "I decided the results were worth any risk," he says.