PEOPLE who feel extremely unhappy for short periods of time may be suffering from a new and worrying type of depression: new because it has just been recognised as a distinct illness and worrying because standard drugs do not seem to alleviate its symptoms.

In recurrent brief depression (RBD) people suffer spells of intense illness that last from three to five days, although some attacks can stretch into a couple of weeks. They can expect to suffer up to 20 short depressive episodes a year.

The signs are more serious than a spot of anxiety or a feeling of tiredness. Symptoms will be so extreme that half the sufferers will feel suicidal. If a standard psychiatric questionnaire is applied, their score will indicate a classic clinical depression. They will complain of being moody, listless and unable to concentrate; they will feel pessimistic, despondent and may have stopped taking proper care of themselves.

Up to 7 per cent of the population may suffer from the illness, experts believe.

Now, after a study involving thousands of patients in 14 countries, RBD has been included in the World Health Organisation's international classification of diseases.

The British consultant psychiatrist who can take much of the credit for putting RBD on the clinical map is Dr Stuart Montgomery, of St Mary's Hospital, London, who has been studying the illness for almost 20 years. For the past 15 years he has been pressing for its recognition. His two papers in 1979 and 1983, together with a more recent study by the Swiss epidemiologist Dr Jules Angst, finally convinced the WHO referees.

Unhappily, it may take longer for the family doctor to recognise this type of depression, although he can do little more than offer support and advice. 'I missed it myself on one occasion and had to have the patient back to apologise,' Dr Montgomery says. 'It does not surprise me that colleagues may not recognise it. But it is fairly easily spotted, although most GPs are unaware of its existence and the majority of sufferers, therefore, go undiagnosed.'

For years psychiatrists have discounted the notion that people can be clinically depressed for such short periods of time. Led by the Americans, they have rejected the idea of any form of depression that did not last for two weeks or more.

Dr Montgomery stumbled upon the illness when he was investigating suicidal patients who appeared to have neither a good reason for their hopelessness nor the expected history of long-term depression.

But many did appear to suffer from short bursts of depression. He discovered literature on the subject that went back 50 years. 'We now have conclusive evidence that patients can suffer from this illness and that it is as prevalent in the population as ordinary depression,' he says.

Dr Montgomery says RBD is a disturbing illness which, although it displays all the hallmarks of classic clinical depression, has so far failed to respond to drugs.

'Patients can feel suicidal; they are withdrawn and they can be both verbally and physically aggressive. The most important thing from the point of view of the patient is to have the disorder identified because then it is possible to take some action to ameliorate any possible effects.'

Dr Montgomery says patients are reluctant to admit they have the condition, fearing problems with work, colleagues and employers. 'I have a fairly good idea that a number of well-known and influential people are sufferers, but it is a question of getting people to go public,' he says.

Drug therapy for the condition has so far proved ineffective. The older anti-depressant drugs have failed to provide a solution. High hopes for the newer types of drug also have not been realised.

'We are looking at other drugs, but in truth we have no answers at present. What we can do is to tell patients to make arrangements that will allow them to cope with the illness when it occurs.

'My advice to them is to take time off work if possible or to avoid meetings that might lead to stress and confrontation. Generally they should put their activities into low key until the symptoms subside. Not everyone can do this, and I had one patient who had to change his job and become self-employed to cope with his depression.'

Dr Montgomery has started another drugs trial in the search for a treatment, but says that more detailed work will be needed before answers are found.

'I believe the chemical and biological composition of this illness is quite different to other types of depression and that is why we are having such poor success with the drugs currently available,' he says.