Tens of thousands of cancer patients in the UK are risking their lives by failing to take the drugs they have been prescribed.

Scottish researchers have found that half of women fail to finish a five-year course of the breast cancer drug tamoxifen - without taking the advice of a qualified health practitioner - and one in five regularly forget to take a tablet.

It is believed patients are being put off by the side-effects, which include hot flushes, sweating, weight gain, period changes, fatigue, mood swings and headaches.

The new research by specialists at Dundee University warns that women who miss at least one tablet every five days have a 10% greater risk of dying.

The study, which followed the prescription records of more than 2000 women, shows that as many as 10% stopped taking tamoxifen within the first year of the five-year course. Some 19% dropped out within two years and 32% stopped within three-and-a-half years.

Cancer Research UK said the results of the study were of "real concern" and urged patients thinking about discontinuing medication to contact their doctor or oncologist, who can prescribe drugs to overcome side-effects.

It is estimated that more than 60,000 patients were prescribed tamoxifen in Britain last year, with nearly 5000 in Scotland.

The investigation, funded by the Medical Research Council and Breast Cancer Research (Scotland), also showed that younger women were more likely to stop taking their medication early but there was no difference between richer and poorer groups of women.

Professor Alastair Thompson, based at Ninewells Hospital in Dundee and the senior breast specialist on the study, said the findings painted a "worrying picture".

Colin McCowan, lecturer in health informatics at Dundee University's community health sciences division and one of the lead authors of the study, said: "Tamoxifen has some side-effects which mean it is a difficult drug for women to take.

"To say to a woman that you have to take medication every day for five years is a difficult thing in itself, but then you have to add to that the side-effects.

"We are not trying to tell women that they are behaving very badly and need to sort themselves out.

"What we are saying is that we understand that this is a problem, and that what they need to do is that if they find they are not taking the medication, or they decide they don't want to take it, it is vitally important that they go and discuss this with their doctors and oncologists, so that they can talk it through."

Dr Lesley Walker, Cancer Research UK's director of information said: "We know that tamoxifen saves lives, so these results are a real concern. It's not disastrous if women simply forget to take the occasional tablet but if they forget regularly and don't complete their treatment we need to know why."

Ironically, a study published in the Lancet found women who take tamoxifen and suffer the side-effects are actually 10% less likely to have a recurrence of breast cancer than women who do not develop these symptoms.