Hundreds of thousands of women taking drugs to reverse bone thinning could be doubling their oesophagus (gut) cancer risk.

A major study shows those taking bisphosphonate (a class of drugs used to strengthen bone) drugs for five years, the recommended duration to improve bone strength, are at highest risk, but any level of use was linked to excess risk.

Around 1.4 million British women are eligible for treatment because of osteoporosis, but a quarter do not respond or suffer crippling stomach side-effects, reports the Daily Mail.

Patients can avoid these symptoms by taking their pills on empty stomach and stay upright for 30 minutes before eating and drinking, say doctors.

It is thought that gastric reflux, where stomach acid flows back up the food pipe, may cause precancerous changes, according to the British Medical Journal.

Researchers from the University of Oxford's cancer epidemiology unit and the Medicines and Healthcare Products Regulatory Agency (MHRA) looked at the use of oral bisphosphonates, including Fosamax, and three types of cancer.

For those taking the drugs for at least three years - five years on average - the risk was more than double compared with those who had never taken them.

Typically, oesophageal cancer develops in one per 1,000 people aged 60 to 79 years over five years. Five years' use of the drugs would push this up to two cases per 1,000.