Half of breast cancer patients are failing to complete their tamoxifen prescription, significantly decreasing their survival chances, a study has found.

Published in the British Journal of Cancer, the study highlights the poor patient compliance rate of taking the drug, which has been shown to increase survival chances if taken for five years.

According to the prescription records of more than 2000 women, 51% failed to finish a five-year course and one in five regularly forgot to take the drug. Researchers at the University of Dundee found that women who missed at least one tablet every five days had a 10% greater risk of dying.

“This study paints a worrying picture,” said Professor Alastair Thompson, a senior breast specialist on the study based at Ninewells Hospital Dundee. “Tamoxifen is prescribed for five years to offer the best chance of surviving breast cancer, and not taking the tablets means that women could be disadvantaged.”

Results from the study, which was funded by the Medical Research Council and Breast Cancer Research (Scotland), also showed that younger women were more likely to stop taking the drug early.

Dr Lesley Walker, Cancer Research UK’s Director of Information, called the results “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.”

This is not the first study that has shown problems with tamoxifen compliance. In January last year, the US journal Cancer reported that almost 25% of women taking the treatment stopped after one year. The main reason given for discontinuing was the drug’s adverse effects, which include mood swings and hot flushes.

Originally developed by AstraZeneca, tamoxifen has been considered the “gold-standard” in breast cancer treatment and while now being widely genericised, the drug is increasingly under threat from the newer aromatase inhibitors.

UK figures suggest only one in six patients taking medications are fully compliant causing drug wastage that costs the UK government £800 million each year.