Find yourself tapping your foot all the time? Or clicking your fingers? Do you get bored easily? And find it hard to concentrate? Do you often make careless mistakes?

These may all be symptoms of the adult version of attention deficit hyperactivity disorder (ADHD). We have become relatively comfortable with the theory that increasing numbers of children may be so boisterous, impulsive and difficult to deal with that they are actually ill (although not everyone agrees on this). But until recently it was thought that the condition, which may affect as many as 5% of British children, disappeared during adolescence thanks to brain development and hormonal changes.

Now, however, some scientists and doctors believe that it can persist in later life. Edwina Mitchell, 38, is one of the few adults to have been formally diagnosed: she was told she had adult ADHD last year. "It was such a bloody relief," she says. "I've got a label now for what was going on. I had no idea what was wrong with me. I was frequently diagnosed as suffering with depression, but when I read about depression the symptoms never quite seemed to fit.

"You can't engage a person with depression, stimulate them or make them laugh. When you've got someone who's got ADHD, they may seem quiet and subdued, but if you can get their attention, you watch the way they come right up. The moment I found something to engage in, like adult education, decorating, sewing or knitting, then I'm happy, right up there."

The first thing you notice about Mitchell is that she talks very fast, and for lengthy periods. "My biggest problem is that people always misinterpret me as being aggressive. That has reduced me to tears on a number of occasions, because I'm not aggressive." She is also a fidget, and constantly plays with a ball of tissue to give her hands something to do. She gets easily bored, finds it hard to remember things, is frequently distracted and needs to write copious lists in order to keep her life organised. Mitchell is impulsive, and tact is certainly not a forte. "I can't help it. I speak the truth. If someone says, 'Do I look a sack of shit in this?', they'll get: 'Yes you do.' You can imagine how many problems that causes."

Like many adult ADHD sufferers, Mitchell has a long history of childhood difficulties. Socially isolated at school, she often played truant and was seen as a troublemaker. Doctors diagnosed depression when she was 12 and by 14 she was expelled and pronounced ineducable. She started to take drugs as a teenager.

"American research shows that children who don't get treatment may as adults self-medicate on illicit substances," says Dr Clive Jones, a neuroscientist at University College London. Mitchell's chosen drug was speed, which has the paradoxicaleffect of slowing ADHD sufferers down. "Speed used to leave me quiet and at peace. People used to laugh: 'Give her some speed and shut her up.'"

Mitchell married at 17, only to divorce and raise her two sons alone. She has never had a job, probably, in part, because she comes across badly in interviews. "I frighten them to death," she says.

Adult education through the Open University has been her salvation. She threw herself into her studies with the type of high energy typical of ADHD people, although she had enormous difficulties remembering information and structuring essays. It was thought she might be dyslexic, and it was during a test for dyslexia that an educational psychologist suggested that she might have ADHD.

"I'd never heard of it," says Mitchell. Nor had any of the mental health services in her area. Her GP was unable to find anyone on the NHS to do a diagnostic test, and in the end her university paid for her to have a private assessment at the Marchwood Priory Hospital in Southampton. Once a positive diagnosis was obtained, her GP was at a loss as to how to help. "He said, 'If you bring me the information, I'll act upon it.'" Mitchell and her partner trawled the internet and came up with a list of drugs used in the US, one of which was Dexedrine, an amphetamine-based drug. "I told him I wanted to try that one - it was what I abused as a teenager."

Dr Brian Toone, consultant psychiatrist at the Maudsley hospital, is one of the few psychiatrists who treats adults with ADHD. He says that medication is the most effective treatment. Most are prescribed Ritalin (methylphenidate hydrochloride), the same amphetamine-based drug used for hyperactive children. It is thought to work by stimulating parts of the brain that are under-functioning.

Amphetamine is very similar in composition to some of the neurotransmitters, such as dopamine and noradrenaline, which occur naturally in the brain. One theory is that children and adults with hyperactivity have an abnormality in the shape of the receptor molecules on the surface of the nerve cells to which the transmitters attach themselves. According to Dr Toone, who has so far seen 200 adult patients, more than half respond well to drug treatment.

Mitchell now takes her medication when she feels under pressure. "I find it slows me down. I can write better essays, and am more able to focus. I feel happier and more contented."

How to tell if you have ADHD

Distinguishing genuine ADHD from normal energetic behaviour is a tricky business. As a guideline, if you answer yes to 15 or more of these questions, there may be cause for concern.

1. I have difficulty getting organised

2. When given a task, I procrastinate

3. I work on a lot of projects but can't seem to complete them

4. I tend to make decisions and act on them impulsively

5. I get bored easily

6. I can't seem to reach my goals, no matter how hard I try

7. I often get distracted when people are talking

8. I get so wrapped up in some things I do that I can hardly take a break or switch to something else

9. I tend to overdo things, even when they're no good for me

10. I get frustrated and impatient easily

11. My self-esteem is low

12. I need a lot of stimulation from things such as films and computer games, new purchases, lively friends, driving fast, extreme sports

13. I say or do things without thinking

14. I like to do things my own way

15. I often find myself tapping a pencil, swinging my leg, etc to work off nervous energy

16. I can feel suddenly depressed when I'm separated from people or things I like to be involved with

17. I see things differently from others, and when someone gets angry with me for doing something that upsets them, I'm often surprised

18. I tend to be careless and accident-prone

19. People see me as "a risk-taker"

20. I make a lot of careless mistakes

21. I have blood relatives who suffer with ADHD, depression, bipolar disorder or substance abuse

• Information taken from Additude Magazine, published by the American support group Children and Adults with Attention Deficit Disorder. Website: www.chadd.org