Last month, an inquest was held into the death of Dewi Evans, a 61-year-old man from south Wales. Mr Evans had been a patient at a psychiatric hospital in Pontyclun and was suffering from pica, a rare disorder which makes the sufferer feel a compulsion to eat non-food items. He had undergone surgery twice before, but died this time, after attempts to remove objects including a screw, a pen top, a magnet and some coins from his bowel. At the inquest, the hospital's deputy manager explained how an extra fence had to be erected around the hospital to stop Mr Evans going looking for things to eat, and how staff had to constantly monitor him in case he tried to swallow objects or drink bottles of cleaning fluid.

Common cravings in people with pica include the urge to eat soil, coal, rust, chalk and paper (in the 16th century it was given its name from the Latin for "magpie" due to the sufferer's often indiscriminate eating), although people have been known to ingest anything from animal faeces to bits of metal.

"Pica usually appears in people of a low mental age," says Gregory O'Brien, professor of developmental psychiatry at Northumberland University. "So it affects young kids and people with severe learning difficulties." Professor O'Brien says 1%-2% of people with learning disabilities suffer from extreme pica. "It's really not very common but when it occurs, it can be bad."

It can cause digestive problems - ingesting soil can lead to worm infestations and damage to teeth. It can also have grave consequences if the items consumed are poisonous or, as in Mr Evans's case, cause an obstruction in the intestines.

In 2002, a 62-year-old French man with a history of mental illness went to hospital complaining of stomach pains. An x-ray showed he had swallowed five kilograms of coins, necklaces and needles; his stomach was so heavy it had been forced down between his hips. He died after an operation to remove the objects. In 2000, Edward Cope, a 33-year-old man with autism from Manchester, died from complications after swallowing 10 buttons, a drawing pin, pieces of chain and bone and a large amount of black foam rubber.

People with pica have to be watched constantly. Vivien Cooper founded the Challenging Behaviour Foundation (thecbf.org.uk) to give parents of children with behavioural difficulties information and support. Her son, Daniel, who is now 21, has Cri du Chat syndrome, a genetic disorder that results in a range of learning disabilities and challenging behaviours - one of which is pica. "There is nothing he wouldn't consider eating," she says. "If we go to the beach, suddenly you'll think 'did he put something in his mouth?'. He has eaten sand, pebbles, stones, cigarette ends, pen tops, the ends of plastic razors, string, Sellotape. When he was younger, he drank a bottle of washing-up liquid. If he cuts himself, we can't put plasters on him because he'll eat them. He lives in a residential centre and recently ate some disposable rubber gloves. When he was younger, he learned that he could rip open a teddy and eat the stuffing."

Because Daniel's pica is inconsistent, Cooper says she, and the staff who help look after him, have to be always on their guard. "We have had to take him to hospital many times, mainly just to check something hasn't got stuck if we suspect he has swallowed something. You have to be constantly aware but in a way that doesn't inhibit his life, and you have to make sure his environment is right - he has laminate flooring in his room and an indestructible mattress."

There is no explanation for Daniel's pica. "He was given blood tests for zinc deficiency but nothing showed up."

In one American study, 25% of patients in psychiatric care were found to have pica and it appeared in 60% of people with autism (pica tends to be a symptom of something else rather than a disorder in itself).

There are two main types of the condition, says O'Brien. "Food pica, where what a person eats is edible but is not prepared for eating - for instance, I have had patients who would eat a catering-sized tin of coffee powder or gorge on marmalade or potato peelings - and non-food pica, where people eat anything else. Once it starts, it can be difficult to control."

Pica can also appear in young children who go further than just putting things in their mouths. It is normal for infants and toddlers to put things in their mouths but 20% of children will have pica (identified by eating a non-food item repeatedly for more than a month) at some point. Most will outgrow it but the disorder is commonly associated with autism and other learning difficulties, or in children who have suffered brain damage. "If it appears in young children, for the most part it improves with age," says O'Brien. "By two, they should really have stopped trying to eat non-food items. Most parents will watch them and try to divert them with something else."

It is also more common in severely neglected and deprived children who do not necessarily have a learning disability. "I have seen children in orphanages in the Ukraine and parts of eastern Europe who had been totally lacking in stimulation and some of them showed signs of pica. They would eat fluff off the floor and blankets, anything for a bit of stimulation," says O'Brien.

Some forms of pica can also be cultural. Geophagia (the consumption of earth, typically earth that has a high percentage of clay) is considered healthy in many places because people believe it can cure diarrhoea, stop nausea and remove toxins (kaolin is a clay mineral often used in commercial diarrhoea remedies). In some countries in Africa and south America, clay is sold to pregnant women as a "medication".

Although little research has been done on pica, many doctors believe that in some cases, it can be a sign of deficiency in some nutrients, especially iron, even when the items eaten do not contain the nutrients needed. Anecdotal evidence suggests that prescribing iron supplements can reduce or eliminate pica. Pagophagia is the compulsive consumption of ice and is thought to be a response to anaemia, perhaps because it soothes a swollen tongue, one symptom of low iron.

Pica can also start in pregnancy. Around 40% of pregnant women experience cravings and a small proportion of these will crave non-food items. Doctors and midwives have reported women craving coal, chalk, even cigarette ash. "There is a theory that it is linked to iron deficiency," says Gail Johnson, education and professional development adviser for the Royal College of Midwives. "For most women who have pica, it comes and goes and it usually only happens in the early stages of pregnancy. On the whole, most women are sensible and stick to an appropriate diet even if they do have cravings for odd things - we certainly don't recommend women go out and eat soil".