Longterm use of drugs that are commonly prescribed for osteoporosis may be doubling the users' risk of developing cancer of the oesophagus, a study warns.
The drugs are routinely used to either treat or prevent osteoporosis and other bone conditions and are taken by many hundreds of thousands of patients.
Research in today's British Medical Journal links the use of oral bisphosphonates to an increased risk of getting one of the more severe forms of cancer, although no links were found to stomach or bowel cancer.
Experts from the University of Oxford's cancer epidemiology unit and the government's Medicines and Healthcare products Regulatory Agency (MHRA) analysed data from a UK GP practice database on around 6 million people.
Among those aged 40 and over, 2,954 had oesophageal cancer, 2,018 had gastric cancer and 10,641 had bowel cancer, all diagnosed between 1995 and 2005.
Examination of their health records showed that the chance of oesophageal cancer was 30% higher in people who had had one or more previous prescriptions for oral bisphosphonates, compared with people who had never taken the drugs.
The risk was almost double for those who had 10 or more prescriptions, compared with those who had had less than 10. And 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 had a prescription for the drugs.
Typically, oesophageal cancer develops in one per 1,000 people aged 60 to 79 over five years. Use of oral bisphosphonates over five years would push this up to two cases per 1,000 people, the authors said.
The main author, Dr Jane Green, said: "Oesophageal cancer is uncommon. The increased risks we found were in people who used oral bisphosphonates for about five years, and even if our results are confirmed, few people taking bisphosphonates are likely to develop oesophageal cancer as a result of taking these drugs.
"Our findings are part of a wider picture. Bisphosphonates are being increasingly prescribed to prevent fractures, and what is lacking is reliable information on the benefits and risks of their use in the long-term."
Each year, around 8,000 people in the UK are diagnosed with the disease and around 7,500 people die from it.
An MHRA spokesman said the findings should not stop patients from taking their bisphosphonate medicine. He said the UK Commission on Human Medicines had advised that the evidence from the study was not strong enough to suggest a definite causal association between oral bisphosphonates and oesophageal cancer. However in order to reduce risk of oesophageal irritation it is important to carefully follow the instructions.
The spokesman added: "Patients should also report any signs of oesophageal irritation such as difficulties or pain on swallowing, chest pain, or heartburn to their doctor."
Recent studies have suggested no link between the drugs and oesophageal cancer, but it is thought the drugs do protect against breast cancer in post-menopausal women.