Ian Allen was in a supermarket in Gloucester when he decided to buy 150 tablets of paracetamol. The sales assistant told him, quite properly, that he was not allowed to sell him anything like that amount. 'But I live miles away,' Allen explained.

'I can't come running here every few days.'

Eventually Allen, who is an eloquent 38-year-old wildlife photographer, persuaded the assistant that he should sell him as much as he wanted.

'Don't tell anyone,' the employee said. 'And don't do anything stupid with them.'

'This was rather ironic,' Allen says now. 'Because that was exactly what I was about to do.'

The brain remains the great unconquered organ of scientific and medical knowledge. Ian Allen is fond of saying that if we knew as little about the workings of the heart as we do about the brain, then nobody would dare to perform open-heart surgery. When the brain malfunctions we are often at a loss to detect why, and we are still groping towards effective treatments. Paracetamol is a blunt tool most often used in the masking of headaches, but Allen's intended use was for suicide. He believes that this was a side effect of his doctor prescribing him a drug known as an SSRI - selective serotonin re-uptake inhibitor - a family of medications once recognised only by the tradename of Prozac, but now also marketed as Seroxat, Cipramil, Lustral, Efexor, Dutonin and Faverin. They are most commonly prescribed as treatments for depression, but each year new applications are being found for them. The molecular shape of the drugs is designed to be highly specific, but they are often prescribed for the most unspecific of symptoms: anxiety, insomnia, shyness, natural sadness following bereavement. The drugs are now so widely used that it is difficult to find any community or large organisation without members who are taking them.

In 2000, just under 12m prescriptions of SSRIs were dispensed by the NHS in England alone, almost 4m more than in 1997. An interesting pattern is emerging regarding their use, quite aside from the question of why we appear to be getting more depressed and anxious. The majority of those on the drugs believe their lives have benefitted from their complex but still unrefined chemistry, but there is also a growing band of desperately unhappy and angry people who claim the medications have all but destroyed them. Inevitably, many solicitors are now involved, and there is the possibility of class actions directed against the pharmaceutical companies who have made the invention of drugs of the mind one of the top priorities of the new century.

Ian Allen says he was given his SSRI for acute insomnia. 'I was a normal person who very rarely visited my doctor. Within a day of taking the drug I was overcome with what I can only describe as an intense disquiet - the most unpleasant thing that I have ever experienced in my life. Many of my friends said they just didn't recognise me.' He says he went back to his GP the next day and told him that the pills were having devastating effects, and his doctor replied that it was unlikely to be the drug. SSRIs are designed to enhance the brain's levels of serotonin, a substance involved in the transmission of nerve impulses and widely thought to be a key element in the maintenance of a balanced mood. They do not generally take effect for two or three weeks, so Ian Allen carried on taking them. He lost almost three stone in three weeks. After a while his employer sat him down and told him he did not consider him well enough to continue working.

'A lot of time on the drugs you feel nothing, but then suddenly the most minor of things can drive you to the most catastrophic actions. In three months I tried to kill myself on six separate occasions, always with lethal intent. The paracetamol tablets. I tried to gas myself in a car, I tried hemlock. Paracetamol is an extremely unpleasant way to kill yourself. It doesn't kill you instantly, and I was found by someone. I ended up in hospital with liver and renal failure.' The strangest thing is, he says, when he woke up in hospital he really couldn't understand why he'd done it.

Allen's recovery began a few weeks after he came off Prozac last August, and he began a lengthy complaints procedure which has yet to yield him any satisfaction. He claims the medical profession and the NHS have brushed him off, blaming his own underlying psychological imbalance rather than the effects of a drug upon it. This is a dilemma encountered by many of those who have bad experiences with SSRIs: because it is so difficult to measure emotional and other mental states, it is almost impossible to show that a worsening condition would not have occurred without interference. The same, of course, applies to an improvement.

What is clear is that by their very nature, anti-depressants tend to be given to people who are in a vulnerable situation. Jenny Stanaway remembers her doctor telling her that she 'needed a little something' to help her through her menopause. She is 52, and used to enjoy a reasonably active life.

She used to work as a cleaner for 20 hours a week, but has not done so since July 2000, which was when she stopped taking her daily morning dose of 20mg of Seroxat.

Stanaway's problem on the drug - severe headaches - was nothing to the predicament she faced when she came off it. 'After 11 months of it I was still getting very bad headaches and I felt the drug wasn't right. My doctor agreed, and she said to come off it by taking one every other day and then stop, which is what I did. After four days I went into withdrawal. It started with leg spasms. I had nightmares. Muscle weakness. My balance went.'

She saw the duty doctor, who told her to go back on Seroxat. She did this, but the symptoms continued. After a further month, she says her regular doctor said she was very sorry about the adverse reaction, and that the withdrawal now seemed impossible to stop. She came off the drug completely. 'The past 20 months have been unbearable,' she says. Her husband asks her to try to remember what she was like before that episode in the supermarket. She has been on incapacity benefit since January, but only wants to get back to work. 'No one knows how to do this. People tell me I'll get my balance back eventually, but I'm yet to see it. I feel I need a miracle.'

Towards the end of last year she saw a newspaper advertisement soliciting for victims of medical negligence. The person she called referred her on to Mark Harvey at the Cardiff firm of Hugh James Ford Simey, who was then unaware of the full extent of the problems linked to Seroxat. But now he is, for he has since heard more than 100 other stories.

Harvey has conducted many class action medical negligence cases during his career, beginning with the claims against Eli Lilley, the makers of the occasionally fatal anti-arthritic Opren in the early 80s. He is currently seeking compensation for users of Lipobay/Baycol, the anti-cholesterol drug pulled off the market by Bayer after adverse reactions with other drugs resulted in a number of deaths. The case of Seroxat is not unexpected, he believes. 'The drugs are all trying to fill that huge gap in the market - covering anything from mild to serious depression - and if you can produce something that alleviates the problems and isn't addictive, then you have a huge winner. People now go to their doctor and say, "But will I be addicted?" because they've all heard the Valium stories.'

The data sheet that accompanies each packet of Seroxat has a bold claim: 'These tablets are not addictive.' A little later in the patient instructions, after information about not taking it with the popular blood-thinning drug warfarin and other medications, the reassuring message appears again: 'Remember that you cannot become addicted to Seroxat.' Many patients now regard this claim as unacceptable.

Mark Harvey says he is still 'shaking the tree' to see how many people are suffering from the sort of severe withdrawal symptoms afflicting Jenny Stanaway. People are learning of his interest at the rate of about two a week. The most common story he hears is that the drug initially worked, but then the difficulties really started. At present he has 120 people on his books, and he has commenced applications for legal aid.

The data sheet supplied to doctors by manufacturers GlaxoSmithKline (GSK) does inform them that withdrawal should be gradual, but Harvey believes that the language employed deliberately downplays the potential problems. 'However you dress it up,' he says, 'they're trying to suggest that it's not a major issue. But I've got people who have been trying to get off it for four or five years and say, "My life is a misery." I've heard this argument about [it not being addictive], but I think it's mischievous. What they're saying is that the body doesn't become so absorbent to the drug that you have to keep prescribing larger and larger amounts. That may well be right. But I have to say that if you're a patient and you read your information sheet that says "These tablets are not addictive," then they understand that as meaning: "If I want to come off this drug then I should be able to do so without any problems, like coming off penicillin." But to say that there's a technical definition to "addiction" is wrong. It's bad enough doing it to a doctor, but you certainly shouldn't do it to a member of the public.'

Harvey is not the sort of hot-headed litigator we may be familiar with from the movies; he does not distrust Big Medicine per se. He acknowledges that a lot of people benefit from Seroxat, and he has a moderate suggestion that falls well short of any grandiose attempt to have the drug withdrawn. 'If [GSK] were sensible, they would sit down and go, "We don't accept any legal liability but we recognise that we could improve the information that we give to the patient and the doctor."'

People become aware of Harvey's involvement principally through the internet, which has recently developed into a vast arena of anti-SSRI campaigning and sad stories from depressives. Websites cater for all types of anxiety and melancholy, and they provide a self-help community for those troubled by their treatments. On the popular 'HealingWell' site, which caters for all ailments, the diabetes message board had recently attracted 406 postings, and the one for multiple sclerosis 261. By the same day in mid-April, the message board for anxiety and panic disorders had received 8,208 and the depression board 9,392.

The postings have titles like 'In A Deep Hole and I'm Sinking Again'. Some consider how to withdraw from SSRIs, but others have gone beyond that. One recent message from Sally186 said: 'I've had a horrible weekend. Been more and more anxious lately - pending divorce and my mother-in-law is dying and I love her and it's too much like when my dad died two years ago. Started feeling horrible suicidal urges late last week and only stayed out of the hospital when my therapist agreed to call me twice a day to make sure I was OK. I don't want to hurt my children. God help me. If I'm not in chat tonight you'll know where I am. Sally.'

There was an immediate response: 'Dear Sally, Your thinking is all negative. You've been handed your fate - you can turn it into something good. It really doesn't have to be as bad as you are making it.'

Depression is not a modern affliction; indeed, it was recognised as a treatable illness by Hippocrates. But only recently have we begun to diagnose the scale of the problem, and only in our lifetime has medical science been able to approximate its biological causes. The World Health Organisation estimates that depression is soon to become the second leading cause of disability - behind ischaemic heart disease and ahead of road traffic accidents. Extensive surveys report that major depressive illness occurs in 3-10 per cent of the adult population, with the prevalence in women two to three times higher than in men. A report published in 1997 suggests that major depression is prevalent in 2.3 per cent of the UK population, and mild depression in 7.7 per cent. Thirty to 50 per cent of cases are believed to go undetected.

By themselves, these figures mean little, particularly to those who are not depressives themselves. In the past decade a few graphic memoirs have thrown light on the true nature of severe illness, and the one thing they make plain from the start is that they are not suffering with a bad, common case of the blues. The American novelist William Styron ventured that 'depression' has been devalued, a word that has 'slithered through the language like a slug, leaving little trace of its intrinsic malevolence'. In his peerless book The Noonday Demon, Andrew Solomon gives an unnerving description of being unable to raise himself from his bed to answer the phone; even a journey to the bathroom becomes a multi-step struggle. On a broader plain, 'the first thing that goes is happiness', Solomon explains. 'You cannot gain pleasure from anything... Your mind is leached until you seem dim-witted even to yourself. If you hair has always been thin, it seems thinner; if you have always had bad skin it gets worse. You smell sour even to yourself. You lose the ability to trust anyone, to be touched, to grieve. Eventually, you are simply absent from yourself.'