Whenever Linda Beaton bends her head forward, she feels a crushing sensation around the middle of her body. Her breathing suffers and she feels nauseous. "It's not exactly something you can avoid, bending your head forward," she says. "So I endure these symptoms every day."

It started with a car accident in 2004. And Beaton, 58, is completely in the dark about what's wrong with her. She has had x-rays, MRI scans and blood tests. She has visited GPs, consultants, neurologists, orthopaedic surgeons, an osteopath, a physio-therapist and a homeopath. She has trawled the internet and called on every medical friend she knows (there are quite a few of them because she was a nurse until she lost her job because of her symptoms). But she remains undiagnosed.

Medically unexplained physical symptoms (Mups) are one of the most common problems in modern medical practice, accounting for somewhere between 15% and 30% of primary care patients. "Without a diagnosis, I can't be sure I'm getting the right treatment and more importantly still, I don't know what the chances are of me ever recovering," says Beaton, who wears a neck collar. "Then there is the sheer exhaustion of trying to find out what's wrong with me. It takes up a lot of time and effort going to see experts and having more tests. I've decided that, for the time being, I actually can't cope with any more doctors. It doesn't help that I suspect that some of them see me as a troublemaker."

She may be right. One study on GPs' attitudes towards Mups, published by Oxford University Press, sums up a typical response from a cynical GP: "I would like to have enough courage to tell the Mups persons that nothing is wrong; you are wasting your time and my time."

Not all doctors are so sceptical, though. The same study found the majority of GPs agreeing that they have a central role in providing reassurance and psychological support to Mups patients, as well as acting as the gatekeeper to secondary care.

Sue Carfrae certainly has no beef with doctors, even though she has no idea why two of her four children - Adam, 20, and Stephanie, 16 - need to use wheelchairs. "They were absolutely normal as babies, but then they started to walk very awkwardly and it deteriorated from there," she says. "Over the years, there have been a lot of tests - from cerebral palsy right through to things I'd never heard of - and there are still tests being done. I can't see that the doctors could have done anything more."

She says that the psychological impact of having no diagnosis takes its toll on the whole family: "Adam is at a residential college where everyone else knows what they've got. Some have life-limiting conditions, and when Adam lost a friend quite recently, he phoned me saying, 'How do I know I haven't got something similar?' I can't answer that, which is awful for a parent. Sometimes his distress has got so bad that he has tried to hurt himself. Stephanie looks at Adam, whose condition is more severe than hers, and has her own worries about that. Then there are my two older children, who also feel cross and frustrated. One ran the marathon for Muscular dystrophy, even though they haven't got that, because he felt he wanted to do something. For me, I think one of the worst things is that there is no support group we can attach ourselves to, where we can get guidance and a sense of belonging."

Liz Swingwood tried to bridge this gap when she founded Swan (Syndromes Without a Name). She can still remember the shock of going with her baby granddaughter, Charlotte, who was experiencing severe developmental problems, to Great Ormond Street Hospital. "The first thing they said was, 'You may never get a diagnosis and there are a lot of you out there.' I was staggered." Ten years on, Swan has 1,300 members, and Charlotte is still experiencing health problems.

Surprisingly, many parents of undiagnosed children are subjected to verbal abuse. "People quite often say things such as, 'What kind of a parent are you? If it was my child, I'd be sure to find out what's wrong,'" says Swingwood. There are also, she says, practical implications for the undiagnosed. "If you can't fill in the box saying 'Condition', you can be refused benefits."

The most common unexplained symptoms are musculoskeletal pain, ear, nose and throat complaints, abdominal pain and gastrointestinal problems and fatigue and dizziness. And there seem to be three main reasons for the lack of diagnoses. First, much as people don't like hearing it, the cause may not be purely physical. With regular codeine buyers, for instance, their headaches are often due to codeine itself. It is also well documented that our internal systems can be affected by stress.

Second, there is a huge amount of overlap in symptoms and conditions. Jane Skerrett, spokeswoman for the National Ankylosing Spondylitis Society, says, "If someone has ankylosing spondylitis [a form of inflammatory arthritis which mainly affects young people], they may visit the doctor with lower back pain around their early 20s. Usually, the doctor assumes mechanical damage. They go back around six months later and they might be sent for an x-ray, but it probably won't show on the x-ray, and they can't be sent for a definitive test because there isn't one." This matters, she says, because patients may be given the wrong advice. "Resting, which is what people are often told to do for back pain, is just about the worst thing you can do for ankylosing spondylitis."

Finally, there are limits to medical knowledge and difficulties can arise in assigning a clear cause to subjective complaints such as pain and fatigue. Julia Smith, a GP, says this isn't something that patients always accept. "They think there's got to be some test out there that will get them an answer. But sometimes there might not be, and it's not because the doctor is incompetent. There just is no answer."

Over-investigation is a concern to Smith. "In some cases, it happens because doctors are afraid of litigation - they don't want to take any chances - but often it's because patients are insistent and I'm not sure that private medical insurance always helps. The patient gets a referral to a specialist, which their insurance covers, and they are met with this old boys' club, where specialists who don't find anything continue referring them on to other specialists they know. I'm not saying it always happens, but when it does it can leave the patient feeling increasingly stressed about the possibility of each condition that is mentioned."

Emma Williams, 36, knows this all too well. Having suffered pain in her legs for just over a year, her list of tests and medical experts reads similarly to Linda Beaton's. "I would find myself having more tests, each of which caused anxiety, but getting no nearer to the cause. When it was suggested I see a neurologist, I really panicked. The specialist said he'd put his mortgage on it not being a brain tumour and that he just wanted to rule it out, but the words had been spoken."

Williams says she is happier having stopped all tests, even though she still has no diagnosis. "The pain is still there, but I've learned how to manage it and that's enough for me."

Some GPs, such as David Beales in Gloucester, are ditching a solely biomedical model for a more holistic approach in the hope that his undiagnosed patients can avoid the excessive physical investigation merry-go-round. When Beales has ruled out any pathological reasons for a patient's pain, he does simple tests using saliva, breathing and heart rate to establish how much the symptoms are related to stress (even pain not caused by stress is usually accepted as being exacerbated by stress). "If they are,I tell patients that while their symptoms are absolutely real, they need to be taken out of the realms of medication into a recovery programme based on health coaching.This might include, for example, exercise, counselling and breathing exercises with relaxation."

For many undiagnosed patients, this could be the only treatment a doctor is able to offer.