Women should be offered 10 years of drugs to prevent the return of breast cancer, rather than five as now, according to a major study showed longer-term treatment reduced the numbers whose disease came back.

Huge numbers of women in the UK and around the world are already taking hormone therapy – drugs that block oestrogen in the body that can fuel breast cancer growth. Many abandon them within the first few years because of the side-effects or because they feel well and do not want to be on medication.

But the study published in the New England Journal of Medicine and presented at a major cancer drugs conference in Chicago could lead to doctors offering aromatase inhibitors, the class of drug taken by most women following their breast cancer treatment, to their patients for an entire decade.

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Most of the 40,000 women a year in the UK with the commonest form of cancer, known as oestrogen receptor positive, are prescribed aromatase inhibitors if they are postmenopausal and the older British drug tamoxifen if they have not reached the menopause. The drugs have different side-effects: tamoxifen can bring on menopausal symptoms such as hot flushes, nausea, low libido and mood swings but aromatase inhibitors can cause joint pain and brittle bones, leading to fractures.

Studies have already shown that women taking tamoxifen for 10 years have fewer recurrences of their cancer than those on the drug for five years. But aromatase inhibitors have been shown to be more effective and are now more widely used.

The study by Dr Paul Goss of Massachusetts general hospital and colleagues was presented at the American Society of Clinical Oncology annual meeting. It enrolled 1,918 women, some of whom took an aromatase inhibitor called letrozole for 10 years, while the rest were given placebo pills. Some had already been on tamoxifen for up to five years. That meant some of the women in the trial took hormone therapy for 15 years in total while others were on it for 10.

Significantly fewer of those who were on letrozole for 10 years had either a recurrence of their cancer or a new cancer in the other breast, although the numbers were small – 67 on letrozole compared with 98 on placebo – which is a reduction of 34%. But 10 years of the drugs made no difference to survival. When cancer did recur, it was usually at stage four and fatal, said Goss.

Goss believes the findings should change practice. He said: “Aromatase inhibitors are now readily available around the world and therefore our results will further improve the outcome of women with breast cancer globally. It will help tens of thousands of women. It will have an enormous impact.”

A further study by the Oxford-based Early Breast Cancer Trialists Collaborative Group, also presented at ASCO, found that there was a real risk of cancer returning up to 20 years after hormone treatment was started. The group analysed the outcomes for 46,000 women from 91 trials who had taken some form of hormone therapy for five years after their breast cancer treatment.

The researchers found that even women with the smallest, least invasive cancers had a 14% risk of recurrence after 20 years and the chances of cancer returning for those with more severe disease rose as high as 47%, suggesting that five years of drug therapy is not enough.

Cancer charities welcomed the findings on aromatase inhibitors, but some experts said women might still choose to stop after five years. “The good news is that we have the data and now women and their medical teams can have an informed discussion,” said Dr Harold Burstein of the Dana-Farber Cancer Institute and on ASCO’s breast cancer committee.

He thought women who were at higher risk of cancer returning because they had experienced more invasive disease in the first place might be more inclined to stay on the drugs for longer. “For lower-risk patients, the benefits are narrow enough that they may opt against it,” he said. In his own clinic, some women have the date when five years is up ringed on the calendar, he said, because they are keen to stop.

The women in the trial who took the drugs for 10 years were probably those who suffered fewer side effects from the start, the paper said.

A considerable number of women stop. A study in Scotland in 2013 found only half of those taking either aromatase inhibitors or tamoxifen were still picking up their prescriptions for the tablets by the end of five years. It also found that those who stopped within three years were more likely to die of breast cancer.

Delyth Morgan, chief executive at Breast Cancer Now, said there was a need for a better understanding of the quality of women’s lives before they were asked to take the drugs for 10 years. “This is a really important study that could one day have a major impact on how we use anti-hormone breast cancer treatments,” she said. “But while this trial showed short-term benefits for patients by doubling the course of standard treatment, we now need to understand the long-term impacts on their survival. How this treatment extension might affect a patient’s quality of life also needs to be investigated before we can consider asking women to live with the side-effects of treatment for an additional five years.”