An Ethiopian schoolgirl called Bira, David Adam begins his story, once ate a wall of her house. "She didn't want to, but she found that to eat the wall was the only way to stop her thinking about it … By the time she was 17 years old she had eaten eight square metres of the wall – more than half a tonne of mud bricks." She had parasites, constipation, a lot of pain and sores in her throat from all the swallowing. "In tears, she walked to her local hospital," where she found one of her country's "eight psychiatrists for a nation of 70 million people", and asked for help.

You can see why a writer would want to start his book with such a story. Girl eats house, it's a brilliant image, as grabbing as Oliver Sacks's The Man Who Mistook His Wife for a Hat. On the other hand, it raises a lot of questions. When did this happen, what was the diagnosis and treatment? How is Bira doing now? Is there anything especially Ethiopian about mud-eating, or to do with being a girl, or a teenager? Was it something smaller and more particular about Bira?

Adam's book answers none of these questions, so I looked up the journal article from which he got the story, which was published in 2008. Eating mud, I learned, is also called geophagia, and is the most common manifestation of a problem called pica, whereby people eat things not usually considered edible. Pica, some think, is more common in people who "live in poverty" and/or the "hot tropics". It can be associated with pregnancy, and/or iron or zinc deficiency, and/or sickle cell anaemia, and/or "neglect".

Bira, however, had not just presented with the pica, but also her obsession with the mud before she ate it. Psychotherapy was indicated, and perhaps, an SSRI antidepressant, but both were too expensive to be on offer. So Bira was treated with clomipramine, an older and cheaper tricyclic antidepressant. She got better, then stopped the drugs and had a relapse, then at the time the article was submitted was thought to be back on them, and enjoying life, and "reportedly symptom-free".

Such an interesting story, and yet Adam tells so little of it. He also makes a curious mistake. Bira, the article says, did not eat "a wall of her house" but a wall described as either "at" or "in front of" her house. It is just not accurate to describe her as the girl who ate a wall of her house. What difference does this make to the story we started with, and how we read the rest of the book that follows?

David Adam's aims are excellent. He wants people to stop talking about obsessive-compulsive disorder as if it were only "a behaviour quirk", and to understand it as the "mental torment" it can be to those who suffer from it: it's not unusual for a compulsive hand-washer, for example, to lose as many as 10 hours a day on their problem – six of them in fretting and four of them at the sink. The book starts with a trenchant dedication: "For those who deserve an explanation", and ends with advice from OCD-UK on how to initiate a discussion with your doctor about a) obsessive thoughts in general and b) obsessive thoughts to do with wanting to hurt a child.

Adam, moreover, has a personal interest in the topic, having had OCD himself for more than 20 years. His particular fear, he says, is that he may have "caught" Aids: from a tissue, a bus shelter, a toothbrush that a friend could have borrowed in that youth hostel in France. In the early 1990s he pestered the National Aids Helpline so often that he had to fake his accent so they wouldn't recognise him. He donated blood just so he'd get the HIV test, but threw away his free biscuit when he found out about the three-month time-lag between infection and antibodies being raised. He's much better now, with his "lifeline" sertraline every morning (clomipramine, he says, is "a beast … with nasty side-effects"; unlike Bira, he has access to a health service that can afford SSRIs) and lots of therapy. But "even on the drugs and after CBT … for most people it's a bit like being a recovering alcoholic … I will probably always have OCD."

So what is OCD, where does it come from and how does it get set off? Well, here is the single most useful fact I learned from this book: OCD is completely different from OCPD, obsessive- compulsive personality disorder, which is simply to be a person with an unusually low tolerance for mess and imperfection – joke-anal people, like Monica from Friends. The need for order and ritual in the lives of OCPD people is "ego-syntonic", odd and possibly anti-social, but simply part of who they are. In OCD people, on the other hand, the thoughts are "harrowing, ego-dystonic", in endless, exhausting conflict with the person's other drives and hopes. It's like having a phobia, but worse, in that you can't avoid it just by avoiding planes or spiders. The stimulus is internal. You generate it yourself.

Is it then a memory problem, that you keep forgetting the switch was off the last time you tried it, not to mention the many times before that? No, the issue isn't with the quality of your recall, but with your confidence in its accuracy, which gets completely lost. And then, when you check it, your confidence drops further, so you check again, and again, and so on, in a vicious cycle. Psychologists, Adam writes, call this sort of thing the "white-bear effect", after the initiation rite the young Tolstoy imposed for membership of his secret society – all you had to do was stand in the corner and not think about a white bear, and "the cursed thing comes to mind every minute," as Dostoevsky wrote in Winter Notes on Summer Impressions. Trying not to think about something once you've started is like "hold(ing) back an avalanche by melting the snow with a candle," as Adam puts it. "It just keeps coming and coming."

A general-audience book about this distressing and debilitating condition is obviously an excellent thing to have, and Adam is a punchy writer. But I don't think he helps himself in the way he tends to conceptualise the problem as essentially mechanical, awaiting only the right screwdriver and a spot of oil: "Consider a personal computer … That is how the mind usually handles thoughts." "The mind is a thought factory." "OCD occurs when the thalamus runs out of control and sends inappropriate instructions back to the orbito-frontal cortex … One way to try to fix the brain is with drugs." To be fair, Adam is only following the current DSM-dominated psychiatric orthodoxy, the main purpose of which, as he acknowledges, is to get private health insurers paying out standard fees for standard diagnoses. And his thinking does get more critical and interesting towards the end: "Labels to some extent are just that, labels." "The categories of the DSM do not ... carve nature at the joints."

A bigger criticism, however, takes us back to Bira and her mud-eating. Why would you cite such a story, yet let inaccuracy creep into its basic architecture? Not deliberately – Adam seems otherwise quite scrupulous. Because you have an unconscious conflict, perhaps, between telling a great story and getting it quite right? Even the scant details provided by the original article's authors suggest all sorts of subtleties to Bira's story, but Adam, for whatever reason, doesn't want to go there. And you can see that conflict at work too in even the short quotes I've given about his own Aids fear. "I do not fear HIV as it is now understood," he explains, at one point, "a fragile, hard-to-catch virus that leads to an infection that is largely managed with drugs ... The HIV I focus on" – adding his "sincere apologies", I'm glad to say, to the people who actually have it – "is the disease of the late 1980s … so severe that in 1986 it demanded the UK government beam into our houses shocking television adverts with crashing gravestones and the catchphrase 'Aids: Don't Die of Ignorance'."

His fear, in other words, seems to be some sort of death-fear, associated with blood and sex and other usual suspects, triggered perhaps by his misfortune in having reached sexual maturity just as an emerging disease became the focus for a massive moral panic. So Adam's Aids fear, too, makes most sense when looked at sensitively and symbolically, as a story. But that doesn't seem to be the sort of story that he wants to tell.