A pharmacist counts pills in a pharmacy. File image: Reuters

Thousands of women are giving up pills that prevent breast cancer in the mistaken belief it is causing debilitating side effects, researchers have warned.

Tamoxifen is offered to women who have either had the disease or are at a very high risk of developing it because of their family history.

The 5p-a-day pills have been shown to reduce the risk of breast cancer occurring by 30% and are prescribed for five years at a time.

But it was found that patients were wrongly interpreting symptoms of the menopause as being related to the drug.

A study of almost 4,000 women led by Queen Mary, University of London found that around a third of them stopped taking the pills before the recommended time.

This included 12% who gave up within 18 months, mostly because they believed they were suffering severe side effects.

The research also studied women who had taken dummy pills and found that they also experienced the same side effects as those on Tamoxifen, including hot flushes, nausea, sickness and some gynaecological symptoms – the main signs of the menopause. Researchers believe many women are wrongly blaming Tamoxifen for normal age- or menopause-related symptoms, which then causes them to stop taking the drug.

Around one in eight women will develop breast cancer during their lifetime, and experts believe Tamoxifen and similar drugs have the potential to prevent many new cases of the disease.

The pills work by blocking the hormone oestrogen, which is known to stimulate the formation of tumour cells.

There are no figures for the number of women taking the drugs but almost 700,000 prescriptions were written for it last year.

Dr Ivana Sestak, from Queen Mary University, said the symptoms reported were largely similar for women taking dummy pills or Tamoxifen.

‘This suggests that women may be attributing normally occurring age-related symptoms, such as those experienced around the time of menopause, to their medication instead,’ she said.

She urged doctors to advise women that some symptoms they encounter could be unrelated to prescribed drugs, particularly if they are expected to experience the menopause during therapy.

The research, published in the Journal of Clinical Oncology, involved 3,823 women from the UK, Europe and Australia who had been prescribed Tamoxifen or a placebo for five years.

The study found that 25% on the placebo medication and 35% on Tamoxifen had stopped taking the pills before the end of the course.

© Daily Mail