Almost half a million healthy women at high risk of getting breast cancer should be offered a cheap course of drugs that can drastically cut their chances of contracting the disease, according to new guidance for doctors.

Hundreds of thousands of women with a family history of the cancer should be offered the £120 five-year "pill-a-day" course of tamoxifen or raloxifene as an alternative to surgery chosen by celebrities including Angelina Jolie and Sharon Osbourne , the National Institute for Health and Care Excellence (Nice) said.

Preventing people from developing the cancer will also save the NHS huge amounts of money treating patients at a time when its budget is under considerable pressure, the health regulator's new guidelines also argue.

Dr Caitlin Palframan, assistant head of policy at Breakthrough Breast Cancer, said the guidance would allow at-risk women to make an informed choice about their future. "We believe that this guideline is a game changer for people with a family history of breast cancer," she said. "In fact for breast cancer overall it's a historic step for prevention. We think more women will have more options to reduce their risk, which ultimately means we will prevent more breast cancer cases."

A previous clinical trial found that tamoxifen, taken for five years, reduced the risk of developing invasive breast cancer by about 50% in post-menopausal women who were at increased risk of getting the disease. Another trial found that five years of raloxifene reduces breast cancer risk in such women by about 38%. This drug is more expensive than tamoxifen but Nice said it would still be a cost-effective option for the NHS.

Tamoxifen has been used for 40 years to treat breast cancer in the UK, whole raloxifene is currently licensed to help treat osteoporosis in women who have gone through the menopause. Neither drug is currently licensed as a preventative treatment in this country, although tamoxifen is used in this way in the United States.

The treatment will be offered to women aged 35 and over deemed to have a high risk (more than 30%) or moderate risk (17% - 30%) of contracting breast cancer.

Professor Gareth Evans, a consultant in clinical genetics at St Mary's Hospital in Manchester who helped to develop the guidelines, said: "Tamoxifen is extremely cost effective, that's because it's extremely cheap.. .and treating someone with breast cancer costs many thousands of pounds.

"So this treatment is potentially not just cost-effective but cost saving to the NHS and more importantly for women they don't have to go through the stress and trauma of a diagnosis, radiotherapy, potentially chemotherapy. So it's a major breakthrough for women that they are going to be able to be offered this treatment in the future."

Prof Evans said women whose risk of contracting breast cancer was 80% or above - like Ms Jolie - may still opt for surgery, which is more effective than drugs. There are also menopause-like side-effects like hot flushes and mood swings but they usually stop after the first six months, he added, with four out of five women lasting the full five-year course.