More than 400 lives and £30 million a year could be saved if breast cancer patients took their full course of drugs, according to research.
Women who fail to take tamoxifen for the full five years have a higher chance of their cancer coming back and suffering an early death.
A new analysis shows patients who do not adhere to their drug regime cost an extra £5,970 on average due to more hospital admissions and needing other medicines.
They also lose an average of 13 months of reasonable quality of life from not taking the once-a-day drug.
Supporting women to take their full course of tamoxifen could save more than 400 lives every year and free up almost £30 million a year - the equivalent of 20 radiotherapy machines.
Colin McCowan, from the University of Glasgow, and his team analysed prescription records for 1,263 women with breast cancer to see how often they took tamoxifen and for how long.
Women who collected fewer than 80% of their prescriptions were classed as having low adherence to the treatment. The findings showed 434 lives a year could be saved alongside millions of pounds if women took the drug for five years.
Writing in the British Journal of Cancer (BJC), the team said: "Patients with low adherence have shorter time to recurrence, increased medical costs and worse quality of life.
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"Interventions that encourage patients to continue taking their treatment on a daily basis for the recommended five-year period may be highly cost-effective."
Around 13,000 women a year are prescribed a five-year course of tamoxifen, usually after surgery, radiotherapy and any chemotherapy.
Side effects include hot flushes, joint pain, fatigue, weight gain and sweats. Rarer side effects include blood clots.
Dr McCowan said: "High adherence to tamoxifen would seem to benefit both the patient and the NHS.
"We want to raise awareness among healthcare professionals that this is a real issue and that women need help and the correct advice to ensure they have the best possible chance of living beyond breast cancer.
"We do know that side effects of this treatment are an issue and we are currently analysing interviews with women to investigate reasons why they do or don't take their medication and other issues around adherence.
"We hope to use these findings to develop interventions to help women and the NHS to get the most from the life-saving drugs that we already have."
In June, it was announced that thousands of women with a family history of breast cancer would be offered tamoxifen to help prevent the disease.
The National Institute for Health and Care Excellence (Nice) said tamoxifen taken daily for five years can cut breast cancer risk by 40% in these women.
Baroness Delyth Morgan, chief executive of Breast Cancer Campaign, which funded the new study, said: "Tamoxifen is one of the most effective treatments for breast cancer when taken as prescribed but sadly some women find it intolerable to take the full five-year course and risk recurrence of their disease.
"This study is a timely reminder that it's so important that women are given support to continue taking their tamoxifen so that they have the best possible chance to outlive breast cancer."
Lesley Smith, consequences of treatment programme manager at Macmillan Cancer Support, said: "Like many forms of cancer treatment, tamoxifen can often have debilitating side effects. Unfortunately, suffering from severe side effects of treatment is not exclusive to breast cancer patients.
"Macmillan estimates that at least half a million cancer survivors in the UK currently face disability and poor health due to their illness and its treatment."