Jim Harris was only 42 when he began to display a disturbing set of symptoms. For most of the day he was irritable. His stamina had virtually disappeared and a constant sense of fatigue assailed him. His sex drive was almost non-existent, he was badly overweight and he was beset by anxiety and depression. He was, in short, a complete mess.

The cause of Harris's complaint was quickly diagnosed, He was suffering, doctors decided, from a lack of testosterone, a hormone deficiency that can turn a normal, healthy chest-thumping Tarzan into a whining, withdrawn wimp.

Nor is Harris's problem unique. More and more men are being diagnosed as suffering from testosterone deficiency, a trend that suggests the male of the species may be just as vulnerable to the vagaries of his biology as a woman is to hers. Irritable male syndrome is here to stay.

For years, women have taken hormone replacement therapy, using patches that secrete oestrogen into their bodies. Now men, particularly in the United States, are flocking to clinics to get testosterone boosts to rejuvenate their flagging sex lives, energy levels and senses of humour. Many - including Harris - swear by the results.

Some doctors now warn, however, that male hormone therapy could have unpleasant and possibly fatal side effects. Just as its female counterpart has been linked to increased risks of breast cancer, stroke and heart disease, testosterone therapy could trigger prostate cancer, as well as increase dangers of blood clotting, which in turn can lead to strokes and heart attacks. Better long-term studies are needed, they say.

One expert, Dr William Crowley of Massachusetts General Hospital, warns: 'I worry that this widespread prescription of testosterone for ageing men is going to precipitate an epidemic of prostate cancer.'

Other experts disagree. 'I think that testosterone therapy not only produces great improvements to patients' everyday lives but will probably prove to have long-term benefits in protecting against diseases like prostate cancer,' said Dr Richard Petty, of the WellMan Clinic, London. 'However, I agree we need better long-term studies of possible side effects.'

Testosterone is a hormone, related to cholesterol, that is made in the testes under the guidance of signals from the pituitary gland. It surges in pulses round the body, controlling male sexuality. Without it, puberty in boys will not take place. It is also linked with sexual performance.

Men with low levels tend to have low sex drives and problems maintaining erections, as well as difficulty in staying awake and fighting fatigue. In later life they also suffer from osteoporosis, which can lead to crumbling of the spine and loss of height.

In general such problems do not affect men until they are over 50 - although increasingly men of 40 and younger are being diagnosed as low testosterone sufferers.

Male animals are suffering the same problem. Gerald Lincoln of the Medical Research Council's human reproductive sciences unit in Edinburgh recently reported that young, testosterone-rich rams will happily head-butt each other all day long but become listless and as nervous as lambs when hormone levels drop. Red deer, reindeer and Indian elephants also behave this way, Lincoln told New Scientist.

In many species these testosterone fluctuations are seasonal, but in others, such as monkeys and primates, they are linked to stress, just as they are in humans. Bereavement, divorce or family illness can send hormone levels plummeting in men. The answer, say medical enthusiasts, is replacement therapy, although they acknowledge that this has not always been a straightforward business.

'A man's testosterone requirements - about 10 milligrams a day - is massively more than a women's need for oestrogen, which is about 100mg a year,' said Petty. 'That makes the stuff difficult to deliver. For example, you cannot use a little drug patch as you do with HRT in women.

'You need a large one the size of a car tax disk to secrete enough testosterone to give a man a sufficiently large daily dose. Unfortunately in about 60 per cent of cases that causes unpleasant dermatological side effects.'

Similarly, pills have been linked to liver damage, while injections tend to produce erratic jumps in testosterone levels inside patients' bodies.

As a result, testosterone replacement therapy has remained relatively uncommon - until now. In the US the pharmaceutical company, Unimed, was recently given approval to sell AndroGel, a colourless jelly that delivers testosterone and is rubbed on the body, usually the shoulders, once a day.

For the first time, hormone-deprived American males can now get enough of the hormonal 'real thing'. And Unimed has certainly shown no signs of hiding its pharmaceutical light under a bushel. The company has begun an aggressive advertising campaign in medical journals in a bid to get the nation's doctors to prescribe its product.

Beside a photograph of a well-built man in his thirties are printed the words, 'improved sexual function'; a happy looking man in shorts on a mountain bike is captioned, 'improved mood'; and a photograph of a policeman directing traffic is labelled, 'increased bone mineral density'.

At the same time, doctors are urged to 'screen for symptoms of low testosterone' and to 'restore normal testosterone levels'.

Now groups of doctors have begun to call for testosterone tests to be carried out on all men over 50 - of whom a 10 per cent are thought to suffer debilitating low levels - with the aim of providing therapy for the worst affected.

A group led by the distinguished Californian endocrinologist, Ronald Swerdloff, recently urged such a campaign, although critics have also pointed out that many members of his group have financial ties to Unimed. Such gainsayers remain dubious about attempts to medicalise male middle age, as they put it, and point to a sea of problems that they say have been associated with testosterone therapy. They point out that known side effects already include a condition called gynecomastia, in which men suffer enlargement of their breasts after taking testosterone. In addition, the testes of those given therapy compensate for the suddenly enhanced levels of testosterone that are pumped into into the body by shutting down their production of the hormone. The result is testicular shrinkage.

On top of this, it is known that testosterone causes the body to boost production of red blood cells, which makes the blood thicker and thicker and raises the chances that a clot will occur. This in turn increases the danger that a stroke or a heart attack will be triggered.

It has been observed that about a fifth of those undertaking testosterone therapy have developed some type of prostate disorder. Links with prostate cancer itself remain inconclusive but worrying, according to many medical scientists.

The US drugs watchdog, the Food and Drug Administration (FDA), has now warned that it has 'issues of concern' about the use of testosterone to treat men whose hormone levels have dropped off merely as 'a normal effect of ageing'. Its director of reproductive and urological drug products, Dr Dan Shames, has also pointed out that the agency never approved AndroGel with a mind to allowing it to be used to treat the andropause (to give the menopause its correct name when applied to males).

Instead the administration expected its use to be limited to men suffering from sexual problems, such as Klienfelter's syndrome or pituitary dysfunction.

On the other hand, the noted physician, Anthony Karpas, in his book Testosterone and the Ageing Male, argues that concerns raised about testosterone's links with risks of heart disease and prostate cancer are simply not supported by hard evidence.

'A recent study, in fact, reported that men with lower testosterone levels were more likely to get heart disease, and while prostate cancer is sensitive to testosterone, the hormone has not been shown to be a direct cause,' he states.

Alvaro Morales of Kingston General Hospital in Ontario, Canada argues in the current issue of European Urology that there is no evidence to connect testosterone replacement therapy in the adult male with the development of prostate cancer.

At present the problem of whether to boost your testosterone levels is unlikely to impinge directly on the British male, however: AndroGel, which is pricey enough in America, is prohibitively expensive here because of import duty: about £5,500 a year per patient. That rules it out for anyone but the odd, libido-limited UK millionaire.

That is unlikely to remain the case for much longer, of course. What happens then to irritable male syndrome is anyone's guess.

Do you need to take a testosterone test?

1 Do you have a decreased sex drive?

2 Do you lack energy?

3 Do you have a decrease in strength or endurance?

4 Have you lost height?

5 Have you noticed a decrease in your enjoyment of life?

6 Are you depressed or irritable?

7 Are your erections less strong?

8 Has your sporting ability decreased?

9 Do you fall asleep after dinner?

10 Has there been a recent fall in your work performance?

Five or more positive answers and you should consult your doctor.