Breast cancer patients are risking their lives by failing to take the tamoxifen they are prescribed, according to a new study published in the British Journal of Cancer.
Half of the women failed to finish a five year course of the drug and one in five regularly forget to take a tablet.
Experts already know that taking tamoxifen for five years increases survival chances and the new research reveals that women who miss at least one tablet every five days have a 10 per cent greater risk of dying.
The researchers - based at the University of Dundee and funded by the Medical Research Council and Breast Cancer Research (Scotland) - used the prescription records of more than 2000 women to see how many did not complete the standard treatment of a tamoxifen tablet every day and linked this to other health records to see if they were more likely to die.
The results show that 10 per cent of women followed for one year stopped taking tamoxifen, 19 per cent of the women followed for at least two years had stopped, 32 per cent of the women followed for three and a half years had stopped and a total of 51 per cent of women followed for five or more years had stopped taking the drug.
The study also showed that younger women were more likely to stop taking the medication early but there was no difference in the rich or poorer groups of women.
Professor Alastair Thompson, based at Ninewells Hospital Dundee and the senior breast specialist on the study, said: "This study paints a worrying picture. Tamoxifen is prescribed for five years to offer the best chance of surviving breast cancer, and not taking the tablets means that many women could be disadvantaged. Doctors and nurses should encourage patients to keep taking their prescribed medications, ensure side effects are managed as best as possible and thus get the maximum benefit from the medication."
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. We need to make it clear that taking tamoxifen regularly for the full five years gives women the best chance of surviving breast cancer. If women are experiencing problems in taking any medication then we urge them to consult their doctor."