Testosterone is the male body’s magic bullet. A decent, naturally occurring dose of the hormone can increase muscle size, decrease body fat, improve stamina and clear up bumps and bruises, while the competitive edge it provides spurs you on to crush your opponents. It is what makes a man a man.

But here’s the rub. A man’s ability to produce testosterone declines from around the age of 30, dropping by about one per cent a year. Lower levels can mean a loss of sex drive, erectile dysfunction, a decrease in bone and muscle mass, even depression. It has been dubbed the ‘andropause’ — androgens being the group of hormones associated with male characteristics. ‘It’s actually a terrible term,’ says Paul Pennington, chief executive of the Andropause Society. ‘But some of the symptoms do mimic those of the menopause:  irritability, trouble sleeping, putting on weight and loss of libido.’

Reversing those symptoms has become very big business. In the US, 5.6 million prescriptions for Testosterone Replacement Therapy (TRT) were handed out in 2011, in a variety of forms: topical gels and creams to smear on to the shoulders, chest and abdomen; an inoculation of oil prodded into the upper buttock every few weeks; and, of course, injections. Global Industry Analysts Inc predicts that sales will triple to $5 billion by 2017 and the number of specialist ‘low T’ practitioners setting up shop in Harley Street suggests that UK men now want in, too.

You can see what’s got their peckers up. Trumpeted as ‘the new Viagra’, TRT promises to reverse erectile dysfunction in seven out of ten men, and boost sexual desire and stamina. It’s a chain that the pharmaceutical companies are all too happy to yank away on. A website hosted by Abbott Laboratories — manufacturer of AndroGel — features an image of a man sitting despondently on a bed, his back to the female partner we can only assume he has been unable to satisfy. Another shows a tubby chap on a set of bathroom scales, his head hung in shame. If he were on TRT, he could expect to see near-instant effects as his body’s metabolism switched from manufacturing fat cells to generating muscle. A study published in The New England Journal of Medicine found that subjects given ‘T’ injections over a ten-week period displayed larger triceps and quadriceps and had significantly upped their weights for bench presses and squats.

The physical boost has obvious benefits for athletes, as does the increased will to win. The notion that TRT gives sportsmen an unfair advantage was at the heart of the investigation into the 2006 Tour de France winner Floyd Landis who tested positive for elevated testosterone after staging a remarkable comeback during one of the race’s more gruelling days. He simply refused to be beaten. But it’s not just the pros. The 17-year-old son of Environment Secretary Caroline Spelman was recently banned from rugby for several months after admitting to taking banned substances, including testosterone.

That aggressive tang also makes TRT a tasty prospect for men who prefer suits to shorts. Senior managers on Wall Street and in the Square Mile are buying into testosterone injections as a way to stay on top and fight off the young guns shooting for their salary. Higher levels of testosterone suppress the production of the stress hormone cortisol; City boys on the ‘T’ report less anxiety and better stamina in the face of rising office hours. Dr Lionel Bissoon, a Manhattan TRT practitioner, says that 90 per cent of his patients are involved in finance; Alpha Dog v2.0 is available for anyone who can afford it.

Because the ‘T’ don’t come free. Dr Malcolm Carruthers, medical director of the Centre for Men’s Health and founder of the Andropause Society, charges £400 for a consultation and another £175 for a blood test. Then there’s £250 for a follow-up consultation and £300 every quarter thereon. That’s £1,725 in your first year of treatment.

There are potential costs to a man’s health, too. According to Dr Jim McVeigh of Liverpool’s Centre for Public Health, it is probable that a man on ‘T’ will experience a huge testosterone crash should he stop the treatment. More worryingly, the quantity of red blood cells in the body may escalate, leading to possible complications such as an increased risk of blood clots in the legs. If you are genetically likely to lose your hair, TRT will jolly that process along for you; similarly it might encourage pre-existing prostate cancer. Remember the acne and oily skin of your teenage years? Higher-than-normal testosterone can bring it back, along with the unexplained aggressive mood swings of a 15-year-old.

‘Testosterone also affects cholesterol balance,’ says McVeigh. ‘You can get a furring of the arteries, posing cardiovascular risks.’ And ironically, taking ‘T’ can recondition you to ‘display some female characteristics’. The sudden surplus triggers the production of extra oestrogen to redress the balance, causing some men to grow female breast tissue. But Pennington has not experienced any side effects. Diagnosed with low testosterone last year, aged 48, he has since lost five stone. ‘Now I run 20 miles a week and feel like I did when I was 30. It’s a good feeling,’ he says.

It is a feeling that more men in the UK are seeking out. From weights rooms to corner offices to the bedroom, TRT is being lauded as the answer to male hormonal recession. Whether the men shooting up will go bust after the boom remains in the balance.