Jane Ginham, a 50-year-old mother from Pembroke, has had the most stressful 12 months she can remember. She was going to move house from Aberystwyth in March, but there were endless complications. The buyer pulled out; her husband had an accident that required him to walk on crutches; a close relative died and there were big problems with the will. 'I just couldn't get up in the morning,' she says. 'I wanted to disappear.' To help her cope, she visited her local health food shop. 'I clearly needed something, but I wasn't going to go on the anti-depressants I'd had before.' She remembered she had once taken St John's wort, a natural herbal supplement, to help her through her backache one winter. 'The young lady in the shop recommended a higher dose than I'd had before, and I bought four bottles.' She started taking three capsules each day. 'It took about three weeks, and then one morning I woke up and thought, "I think I'm ready to face the day". It may just be psychological, but I'm really quite happy now.'

Jane Ginham has one man to thank for her newly sunny disposition, but it's no one she knows. Kurt Amoth is a 28-year-old herb and leaf trader from Salem, Oregon, who spends his days driving around in a green pick-up truck listening to Aerosmith and AC/DC. He dresses in jeans and check shirt, and has short, neatly parted brown hair and an intelligent and outgoing manner. When it comes to sports, he likes Nascar motor racing. His big thing is peppermint and spearmint, and he supplies 80 per cent of America with its mint tea. He used to be in insurance, but he joined his father's company, Aromatics, five years ago as sales manager and did so well that he is now general manager. One of his coups was spotting a new market for a natural anti-depressant that was primarily available only from Chile, Argentina, Bulgaria and Poland, and a few years ago he set about persuading local farmers that they should abandon their traditional wheat and carrot seed and start growing something they'd never heard of.

'The growth in demand was astronomical,' he says of the great St John's wort boom years of the late Nineties, the years in which hysterical reports first appeared in the press heralding 'a natural Prozac'. Demand outstripped supply 10 to one, and it took Oregon's farmers about a minute to realise there might be very good money in it.

Amoth remembers those pioneer days as he drives towards Madras, a small town two hours from Portland where he has 15 acres of St John's wort, enough for about 20 metric tons of fresh flowers and roots. This should keep Britain happy for about a month. He has other fields elsewhere in the state, and one in Washington, but the Madras field is the last to bloom this year. It is the end of July 2001, about a month after it normally begins to flower (the name derives from its regular bloom date of 24 June, the Feast of St John; wort is Old English for plant). He drives beneath the shadow of Mount Hood, the tallest mountain in Oregon, still capped with snow. He passes many wild deer and towns called Zig Zag and Boring ('Oprah did a special from there!'), and an Indian reserve at Warm Springs. He smells his crop as soon as he sees it - a sweet and spicy tang like American mustard. The field is a vibrant yellow, a dense clump of foliage two foot tall topped with five-petalled flowers and an overspill of stamen. The petals are speckled with tiny holes smaller than a pinprick, a feature which provides its Latin name Hypericum perforatum. And there is something else: the field hovers and hums. There are bees on every flower, perhaps 10,000 in all, their sacs bursting with pollen.

Amoth ventures in. 'They won't sting you - too busy.' He tweaks off a few stems, squeezing the buds until their crimson juice stains his fingers. 'They call this the blood of St John,' he says. The tips contain the active ingredient hypericin, the substance which, when combined in a pill-making factory with dicalcium phosphate, microcrystalline cellulose, vegetable source magnesium stearate and Colloidal silicon dioxide, will tend to help many people forget their troubles this gloomy winter. Amoth reckons that the crop will be ready to cut in two days, when it will form part of the tons of St John's wort cultivated in the United States this year, a quarter of the 800 tons produced worldwide. A recent survey suggested that 1.5 million Americans use the herb regularly and that five million others have tried it within the last five years.

On the journey back to Portland, Amoth stops at a health food and vitamin store to examine their stock. There are four different brands of St John's wort capsules, each with two strengths - 0.15 per cent hypericin and 0.3 per cent. There are St John's wort teas and oils and scrubs.

'The consumer is confused,' he says. 'When you have a sudden increase in demand, you can't but help have some very low-quality product out there. There's no standardisation, and the consumer is the loser, and doesn't know whether product A is good or product C is good. If you sell St John's wort flowers, which is the absolute premium product, or St John's wort stems, which is the lowest grade product - those two products could end up on the same shelf right next to each other. One will be two dollars, and one will be 10 dollars, and guess which one the consumer's going to buy? And guess which one isn't going to work?'

Amoth stops at a fuel station to fill up, and comes back from the check out with an elaborately designed bottle of a sweet orangey drink called SoBe Wisdom that contains tiny but modish additions of St John's wort and ginkgo biloba intended to 'promote calm and focused thought'. He drives on, past acres of Oregon grass and onion seed, but then stops suddenly and reverses. He gets out of the car, plucks something from the gravel at the side of the road and climbs back in carrying familiar stumpy yellow flowers. 'You find St John's wort growing everywhere,' he observes, explaining that the problem used to be not how to cultivate it but how to control it. 'It's basically a noxious weed.'

St John's wort is a medicinal product that you keep in the kitchen, next to the ginkgo biloba (to boost memory) and the echinacea (to stave off colds) and the garlic (to lower cholesterol). It forms part of a huge boom in food supplements and self-medication that began in the Eighties but exploded in the mid-Nineties, and has expanded into a multi-billion pound industry (in the United States last year, the market in natural remedies was valued at more than $30 billion).

Why this should be is an intriguing question, for not so long ago the same products were defined as quackery. These days, the appeal of herbal supplements is bound up with our desire for things organic, and our will to improve our health without sitting in waiting rooms. It is an appeal partly grounded in fear, a mistrust of the chemical alternatives produced by Big Science in the pursuit of Big Profits. (But there is irony here, for it is big science that has confirmed the benefits of calcium for osteoporosis and folic acid for neural defects, and aspirin was the valuable result of making a synthetic form of salicylic acid from willow bark).

Self-medication is a boom that is subject to the vagaries of fashion: the evening primrose oil on the breadbin occupies the spot where the royal jelly and the melatonin used to be. Unlike pharmaceutical drugs, St John's wort and its cousins are not licensed by the UK Medicines Control Agency or the American Food and Drug Administration, and the claims that can be made for them on their labels are strictly limited. Their continued popularity depends on unofficial endorsement - favourable but inconclusive small-scale medical studies, supportive coverage in magazines and newspapers, special offers in health-food stores, word of mouth. In some cases, the word of mouth has been gathering momentum for 2,000 years.

St John's wort is an ancient herb, and was known to the Greeks and Romans as an astringent to be rubbed on wounds. Its use has expanded considerably since Pliny the Elder made the first known reference to it in the first century AD, when he described how it cleared bladder troubles when taken with wine. In medieval texts it was referred to as St John's Wurt or the Dutch St Johans Kraut, and in 1619, A New Herbal or Historie of Plants noted the following virtues: 'Is right good against the stone in the bladder...cureth the pain in the haunches...and thrown upon wounds, and naughty, old, rotten and fettered ulcers, cureth the same.'

In 1800, further herbal uses were isolated by John Chambers, a doctor from East Dereham, Norfolk He recommended primrose root for nightmares, male peony for the nervous complaint Blue Vomit, and stinking horehound for palpitations and tremblings. St John's wort was good for poor appetite and the spitting of blood, as well as jaundice(take a strong infusion with columbine and little hairy speedwell three times a day) and worm fever (take with powder of rue and walnut leaves). And then he noted this: 'A tincture of the flowers, made strong in white wine, is recommended greatly by some against Melancholy; but of these qualities we speak with less certainty, though they deserve a fair trial.'

In today's boardrooms in Bohemia, New York, dealing with the Blue Vomit is not uppermost in people's minds. Bohemia is home to NBTY, a company that sells over 1,500 nutritional and natural remedy products - vitamins, herbs, minerals, body-building supplements, dietary regimes - and this year hopes to achieve one billion dollars in turnover. NBTY (its name is an abbreviation of Nature's Bounty) is one of the leading food supplement producers in the world, supplying thousands of Rite Aid and Duane Reade drugstores in the United States. It is the parent company and principal supplier of Holland & Barrett's 461 British and Irish shops. And it is where Kurt Amoth's dried St John's wort ends up to be made into easy-to-swallow plastic capsules.

A few weeks ago, NBTY's executives sat around a meeting room drinking Snapple, and considered how best to sustain the boom in natural remedies, and in particular the market for St John's wort. While they were talking, a woman came in with a list of the company's most exciting new products, the ones that were currently flying off the shelves. These included glucosamine sulphate (a cartilage lubricant derived from crab shell which many people swear helps their arthritis), banana flavour whey protein powder (a dairy-based product recommended for the maintenance of muscle tissue), organic flaxseed oil (providing Omega-3 and Omega-6 fatty acids for greater energy and cell maintenance) and EPO/soy isoflavones (naturally occurring compounds that help reduce hot flushes and night sweats in menopausal women).

'I think glucosamine is currently outselling vitamin C,' says Jim Flaherty, vice president of marketing at NBTY. 'People are taking a more proactive stance when it comes to taking care of their health, and in the control over what they eat. They're not waiting for themselves to be sick. It used to be a fad years ago, but now it's becoming a lifestyle. Especially among the baby boomers. We're concerned that we're not going to be around forever like we thought.'

Flaherty is 44, and has been with the company for 23 years. He takes a lot of his product home. 'I take a multivitamin, I take a separate vitamin E and C,' he says. 'I'll take zinc. The B complex vitamin is important for me because I don't eat enough foods rich in B complex. If I don't take it for a few days I notice a real difference in my energy levels and my mood changes. And vitamin E . If you're a man and you don't take vitamin E then I think you're out of your mind. That's for your cardiovascular health - it's such an easy thing to take and it has so many benefits. It was only discovered a few decades ago, and it's now one of the leading supplements.'

'It's a great product if you're working out like me,' says Scott Slade, an NBTY salesman. Slade, who is 27, worked in one of his company's Vitamin World stores for two months as part of his training. 'It was amazing to see how many young people are coming in buying supplements, be it the herbals like echinacea or St John's wort, or the sports nutrition or even a multi. I was at a baseball game recently and Sarah Jessica Parker is there, and during the game she starts opening this jar and starts popping vitamins, so that's pretty mainstream.' Slade then asks himself a question. 'When you were growing up, would your parents ever give you echinacea? No - that would be for hippies.'

Jim Flaherty enjoys taking visitors on a tour of his site, not least because it dispels instantly any notion that the production of nutritional products is a random, haphazard affair conducted by New Age longhairs. Bohemia is a leafy, lightly industrialised part of Long Island, and the NBTY works occupy several acres with their gleaming chrome machines and technicians in labcoats. There is an analysis wing, where the fresh material is received and measured for purity and potency; there is the soft-gelatin capsule-making unit, which its manager claims is the most advanced in the world, and there is the packaging unit, in which pills are potted and labelled and boxed. In one room, Flaherty points to a silver machine making six million St John's wort capsules in 18 hours; in another, there are large piles of brown boxes awaiting bottling, each holding 25,000 capsules.

'It's pretty much a normal day,' Flaherty explains.

For commercial reasons alone, the multinational pharmaceutical companies are publicly suspicious of the sustained interest in herbs and other natural remedies that have allowed companies like NBTY to invest in such costly machinery. They like to point out that almost every capsule of St John's wort or ginkgo biloba contains, in addition to the natural plant, strictly formulated factory-made chemicals. But privately they are worried about the trend, which is why GlaxoSmithKline and Bayer are now researching nutritional health supplements themselves, despite the impossibility of obtaining patents for herbs and the millions they must invest in safety tests to protect their brand names.

Jim Flaherty's tour includes a visit to the office of Ona Scandurra, the nutritionist in charge of labelling at NBTY. Scandurra says she takes calcium soya, echinacea and glucosamine sulphate, all of which help her active lifestyle and stressful, delicate job. Her office contains a filing cabinet stuffed with research and testimonials promoting St John's wort, and stacks of test certificates verifying such properties as microbial limits, disintegration and dissolution. She talks of a period of 'quarantine', during which the dry herb is checked to contain the 0.15 or 0.3 per cent of hypericin that its suppliers claim. Like many in her company she believes that St John's wort can be highly effective for some people, and is disappointed that all she can put on her American labels is 'Helps support a positive mood', which sounds vaguely like something out of Mary Poppins. In Britain, the NBTY labels are even less helpful to the customer: 'Recent scientific research has made St John's wort one of Europe's most popular products.' This is the equivalent of a car manufacturer saying, 'we sell a lot of these cars', without talking about its cool styling or collapsible rear seats or speed from 0-60. 'We have a whole substantiation file so that we can back up our claims with medical research,' Scandurra says, 'but it's not enough.'

The largest proportion of her research comes from Germany, where St John's wort is widely prescribed by doctors and outsells Prozac by 12:1. It was an overview of German research published in the British Medical Journal in August 1996 that kick-started the strongest wave of St John's wort mania, its findings widely reported in the media. The report analysed the findings of 23 randomised trials involving 1,757 outpatients, all of whom had reported mild or moderate levels of depression.

In 15 studies, the overview suggested that patients fared much better with St. John's wort than with a placebo. In eight studies comparing St. John's wort with standard anti-depressants, its efficacy was slightly higher. Significantly, the dropout rate from the trials was lower among those using St. John's wort than among those using conventional drugs, and lower even than among those who received placebos, which suggested that its side-effects - mild stomach upset, allergic reaction and fatigue- occurred very infrequently.

But these studies were not 'double-blind' studies, the benchmark for clinical research (trials in which neither the patient nor doctor know who is taking what until the results come in), and admitted the possibility of skewed results. The samples were small, and did not test how St John's wort reacted with common prescription drugs. But the great sales boost that followed publication of the BMJ survey suggested that people wanted to believe in the ancient herb, and were keen to try it. Most of those who benefited reported that it only worked after several weeks of continued use.

Sales of St John's wort are now considerably below what they were at their peak two years ago, partly because some who tried it found it to be ineffective, and partly due to three pieces of unflattering publicity. The first emerged last year, when a report suggested it should not be taken by pregnant women as it may interfere with the development of the foetus. There was no firm evidence that this was the case, but also no proof that St John's wort was entirely safe; understandably, many women stopped taking it. In February, two studies showed that it did interfere with a protease inhibitor drug given to Aids patients, and also with a drug administered to patients undergoing organ transplant. These reports suggested that it may also be damaging in combination with many other medications.

The third piece of research emerged in April. This involved 200 patients in teaching hospitals throughout the United States, and showed that St John's wort was ineffective against severe depression. The director of the study, Dr. Richard C. Shelton, a professor of psychiatry at Vanderbilt university, described the results as 'remarkably negative...Since we know there are treatments out there that do work, I would recommend that folks choose an active treatment rather than St. John's wort'.

'The fact that the study was partly financed by Pfizer (the company that makes Prozac) may not have been entirely coincidental,' Flaherty suggests. 'It's hardly ever been suggested that it's good beyond mild-to-medium depression.'

The most significant piece of research into St John's wort has yet to appear. This is a long-term study conducted for the National Institutes of Health that was due to have reported weeks ago. The trial involves many hundreds of patients taking St John's wort, a placebo and another man-made anti-depressant. It is unlikely to provide the last word on the subject, but its findings will have a huge effect on the alternative health industry either way. If the results are negative, our new belief in many other ancient remedies will also be shaken. If they are positive, we will still have no idea how St John's wort works.

Villiers Street is a shabby thoroughfare between Charing Cross and Embankment, and its Holland & Barrett store picks up a lot of commuter trade. Kurt Amoth would be pleased to hear that its manager, Carlos Aguirre, says that his St John's wort is still among his most consistently asked-for products. 'We sell about 30 bottles a week, and the feedback is very good,' he claims. 'One woman comes in every month and buys a very large amount for all her family abroad.' Various forms of the product are on special display, and the bottles have an appealing modern brand name - Herb Tech - which may have been chosen specifically to reassure customers that they're not buying some grainy old hokum collected by a spaced-out student in sandals. Having crosssed coasts and oceans, and undergone rigorous testing, processing and marketing, maximum strength capsules cost £8.99 for 100.

Holland & Barrett hopes to sell 400,000 units of its various St John's wort products this year, but this estimate will change as soon as the American trial results are published. 'It appeals to the public because you don't have to go to the doctor,' says Sharon Flynn, Holland & Barrett's publicity officer and nutritional advisor. 'And it appeals to a wider group of people, perhaps those who are just going through a bad patch or who have SAD [seasonal affective disorder, the common winter complaint brought on by lack of sunlight]. These people might not normally consider themselves ill or clinically depressed.' In Salford, Mary Owens has been taking St John's wort for 10 years, long before it became fashionable. Her supplies then probably came from South America. She used to be a country and western singer, but developed problems with her larynx. Then there was an awkward neighbour, and pressures from her job as a mobile domestic carer - all problems that contributed to her depression. 'I first read about this new thing in the National Enquirer,' she says, 'and it sounded like it would be for me because I've always liked natural products.' She took the herb for several years, but then stopped for a month when she felt everything was going well. She soon felt down again, and went back on it. Now she feels fine on one capsule each morning.

She says she is aware of the health warnings, but isn't 'stupid enough' to mix St John's wort with prescription drugs, and has experienced no side effects. She likes to spread the word. 'A lot of people are taking it around here. I always try to help people out, and if they're feeling stressed or unhappy I tell them about St John's wort and what it's supposed to do.'

Perplexingly, this is all Mary Owens can say for certain - 'supposed to'. More than 2,000 years after the beneficial effects of St John's wort were first noted, we are still unsure about why it acts in the way it does, and what its long-term impact on an individual may be. We are nervous that recently approved synthetic drugs may upset its usefulness, and we are reliant on scraps of contradictory research. Perhaps Kurt Amoth has been wasting his time in Oregon; perhaps he is the uncrowned hero of the day. For £8.99 we are left to conduct surveys of our own.

Supplementary benefits
What the stars are popping...

Antony Worrall Thompson
Seaweed, garlic,
vitamin C, vitamin B, cod liver oil,
selenium, zinc, chromium

Madonna and Halle Berry
use Tony and Tina lipstick which contains St John's wort

Charlene Spiteri of Texas
Starflower oil

David Baddiel
Chinese herbs from Dr Deng

Amanda Donohoe
Blue green algae

Lisa Stansfield
Udo's oil

Terry Wogan, Alan Titchmarsh
fish oil

Vonda Shepherd from Ally McBeal
Sun chorella

Joan Rivers
Everything

Matthew Kelly
Ginseng

Claire Sweeney
Blue green algae, echinacea, cod liver oil

Sheryl Gasciogne
Kelp, cod liver oil, starflower, sea algae, COQ10, vitamin E