Researchers found breast cancer patients prescribed the drugs tamoxifen or aromatase often put their lives at risk by ending the treatment early.
At least half of women given a five-year course in the Tayside area either stopped taking or took markedly less than their prescribed doses from the third year onwards.
Women who followed their treatment regime for up to three years but then took fewer than 80% of their pills in year four and five were at a higher risk of their cancer returning than women who took more than 80% of the prescription over each of the five years.
The findings were made by Colin McCowan from Glasgow University and a team at Dundee University in a study funded by Breast Cancer Campaign and published in the British Journal of Cancer today.
Researchers studied the details of 3361 women in Tayside who were prescribed anti-hormone treatments tamoxifen or aromatase inhibitors for five years after their initial treatment.
This is the current standard treatment for the most common type of breast cancer, oestrogen receptor positive, which accounts for around 40,000 of the 50,000 women diagnosed with the disease each year in the UK.
The women in the study started taking the medication between 1993 and 2008. Using prescription records the team calculated how closely women had followed the one-a-day pill regime and for how long over the five years.
They found that during the first year of treatment women collected, on average, 90% of their tablets.
Over the next three years this figure dropped to 82%, 77% and 59% respectively. By year five half of the women were collecting 51% or less of their prescription.
The study confirmed women who collected less than 80% of their prescription over the five years were more likely to die earlier than women who collected more of their prescription.
Mr McCowan said: "This study shows us it's vitally important breast cancer patients across the UK follow their prescribed treatment regimes on a daily basis for the full five-year period.
"We're now looking at why women are finding it harder to take medication for extended periods of time and we do know side-effects can be a real issue for women on long-term treatments such as tamoxifen. This is why women need the support of their clinicians so they can discuss any problems they are having rather than stopping taking treatments."
Baroness Delyth Morgan, chief executive of Breast Cancer Campaign, said: "We need to give women clear, helpful and timely communication as well as the best clinical support we can, to find ways to manage what can often be extremely difficult side effects and make it easier for them to continue to take treatments."
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