Long-term use of bone-strengthening drugs - used to treat fractures - may boost the risk of oesophageal cancer, Oxford University research suggests.
The study of 3,000 people found taking bisphosphonates for five years upped the risk from one in 1,000 to two in 1,000 for 60 to 79-year-olds.
The researchers said the risk was small, but reliable information on risks and benefits was needed.
But experts said for many, the case for taking the drugs "would be strong".
The findings, published in the British Medical Journal, were based on an analysis of anonymised GP records.
They contrast with previous research which found no increased risk for the bone-strengthening drugs.
'Talk to doctor'
It is not known why the risk may be increased, although the drugs are known to irritate the oesophagus.
Thousands of stomach and colorectum cancers were also studied, but no increased risk was found.
Lead researcher Dr Jane Green said even if the findings were confirmed by other studies "few people" taking bisphosphonates would ever develop the cancer.
"Our findings are part of a wider picture. Bisphosphonates are being increasingly prescribed to prevent fracture and what is lacking is reliable information on the benefits and risks of their use in the long term."
About 600,000 people in the UK are currently taking the drugs - including a tenth of all women over the age of 70.
Dr Laura Bell, from Cancer Research UK, agreed the findings should be treated with caution, pointing out the risks were "still small".
"Anyone who is taking these drugs and is worried about their risk of cancer should talk to their doctor."
The National Osteoporosis Society said: "It is a case of balancing the reduced risk of fractures against the side effects of treatment.
"When you consider the fact that there are 230,000 osteoporotic fractures every year in the UK and 1,150 hip-fracture-related deaths every month, the case for treatment is strong."