It is increasingly clear to me that my toddler lives in a perilous, experimental, illogical world. Her rules are not mine. Indeed, she may not have any rules at all. She is subject to sudden mood swings. She often doesn't listen when I tell her things. She has a short attention span. She is, as far as I know, a normal, healthy 15-month-old child.

These behavioural traits can, however, in the extreme, be signs of a psychological disorder called attention deficit hyperactity disorder (ADHD). In very young children the dividing line between an irrational and difficult toddler and one with actual psychological problems must, then, be pretty blurred. For this reason, a study published last week in the journal of the American medical association makes mind-boggling reading.

Researchers at the university of Maryland found that in the US many children as young as two years old are being prescribed mind-altering drugs for disorders such as hyperactivity, depression, insomnia or obsessive-compulsive disorder. And this trend is growing fast. The team found that between 1991 and 1995 the number of psychiatric drugs prescribed to these pre-school children rose by 50%.

So what's going on? Are American parents becoming more neurotic? Or are psychological illnesses in young children on the increase? And are things going the same way here?

Certainly, the trend towards giving children drugs for psychiatric reasons is not confined to the other side of the Atlantic. In Britain there was widespread concern last year when figures were released showing that prescriptions for Ritalin, used to treat children suffering from ADHD, had shot up. In England the number of prescriptions for this drug rose from 3,500 in 1993 to 126,500 in 1998. This increase has been so steep that it provoked the international drugs control board, a United Nations agency, to issue a warning last year that doctors in the UK are prescribing too much Ritalin to too many children.

But at least the vast majority of children in Britain who are taking Ritalin, or other psychiatric drugs, including the anti-depressant Prozac, are of school age. We are not - yet - doling out mind-altering pharmaceuticals to toddlers. Well, not in any significant numbers, anyway.

According to Professor Peter Hill, consultant child psychiatrist at Great Ormond Street hospital in London, psychiatric drugs are rarely prescribed for under-5s because "pre-school children show a variety of emotional and behavioural problems, most of which relate to their living situation." It is, he says, unwise to label a pre-school child as depressed, or hyperactive and use drugs to treat this. "I would be very, very cautious before deciding to make a diagnosis like depression, ADHD or obsessive compulsive disorder. And even if I thought a four year old was genuinely primary ADHD I wouldn't leap in with medication."

Dr Rea Reason, educational psychologist at Manchester univeristy, who chaired the British psychological society's 1996 working party on ADHD is similarly cautious: "Its nature and its definition keeps shifting." It is a "package of difficulties" and "diagnosis is a quagmire, at any age." It is, she believes, far more important to understand the child, than to label the disorder. "We need to examine different influencing factors for a child's behaviour," says Reason. There are "ethical issues to using medication to control childrens' behaviour."

And apart from the difficulties of "diagnosing" a toddler's erratic behaviour, there's also the fact that the drugs themselves may not work. "There is no evidence to show that psychotropic treatment in very young children actually does any good," says Malcolm Lader, professor of psychopharmacology at the Institute of Psychiatry. In fact, these drugs might actually do a child long-term damage. "If it was a grandchild of mine," says Lader, "I would not prescribe it. The human brain at this age is still developing and maturing and there have been no satisfactory clinical trials for the use of drugs like Ritalin on young children." Indeed, when in one study last year, researchers gave Prozac to baby rats of the same age as pre-school children, the drug produced long-standing changes in the brain.

There are, of course, guidelines as to how drugs such as Prozac and Ritalin should be used. Anyone prescribing these, or other such drugs to children, would have to justify their decision by citing a "reasonable body of medical evidence" if anything went wrong. These guidelines say that Ritalin should not be used on children under six and that Prozac is not recommended for children under 16. But strictly speaking there's nothing to stop a GP, paediatrician or psychiatrist from putting your difficult three year old on Prozac. In fact a recent government green paper on reform of the mental health act proposed that doctors should be able to drug children without parental consent if they have "any disability or disorder of the mind or brain, whether permanent or temporary, which results in an impairment of disturbance of mental functioning."

For some parents, of course, consent isn't an issue. They're desperate for anything that will help "normalise" their child. Most parents will at some point have watched in horror as their little force of destruction barrels through a room, rampages through a jumbo jet, or disrupts a checkout queue. And most of us have probably, in our darker moments, longed for a drug that will silence a hysterical, insomniac child. But in practice, few of us would resort to medication, least of all medication that might harm our child.

But then, this is probably because few of us have any concept of what it's actually like to bring up a child with what Peter Hill calls "a mental disorder." Gill Mead, founder of the ADHD family support group who talks to thousands of distressed parents a year, is adamant that the drugs can and do work.

Consider, for example, Joe. He is three and has already been expelled from two nurseries for hitting and biting the other children. He screams ceaselessly if his mother is out of his sight. He is hyperactive. He will tell his mother to "Fuck off, you bitch" (a sign, Mead believes, of Tourette's syndrome, often associated with ADHD). A spider, a trip to Tesco, even a loud noise can render him hysterical for hours. Mead believes that Joe is suffering from ADHD. "Ritalin would work," she says, "without a doubt."

But when, exactly, does a toddler leave "difficult" and become "disordered"? Critics of the drug trend believe that the rise in prescriptions for under-fives in the US can be put down to parents being unable to cope with difficult children, and not to a rise in mental disorders or the diagnosis of them. "In the US there is gross over-prescription in all children, for what is normal, boistrous behaviour," says Malcolm Lader.

Deborah Lobe, policy and information manager at the childrens' mental health charity Young Minds, believes that this is unlikely to spread to the UK: "The system here works well for under-5s at the moment." GPs, she says, would not prescribe drugs like Ritalin. They would refer the child straight to a paediatrician. Waiting lists, which can prove so destructive for parents of older children who might wait as long as a year for psychiatric referrals, are short for under-fives.

The youngest child she has recently come across with ADHD is 15 months old. He is already on Ritalin. That's the same age as my daughter. It seems inconceivable that anyone could diagnose my child, at this age, with any kind of psychological disorder. But then, she's not banging her head against things, compulsively, all day long; having to wear a crash helmet, and never sleeping.

But are drugs like Prozac and Ritalin really the answer? The researchers in the American study say there is a growing acceptance of drugs as a quick fix for parents who are worried about their children. Whether or not this proves true, what's clear is that alarm bells should be ringing. The last thing anyone wants is for medicating to replace parenting.

Information and advice: Young Minds (helpline for worried parents): 0171 336 8445 ADHD Family Support Group: 01373 826045 Websites: www.pharminfo.com (general information about drugs) www.breggin.com (The International Centre for the Study of Psychiatry and Psychology - info about psychiatric drugs and children). www.p-a-r.org (Parents Against Ritalin) Books: Ritalin Nation - Richard DeGrandpre, WW Norton & Company Ltd, 1999 Ritalin is Not the Answer - David Stein, Jossey-Bass, 1999