Morphine, commonly used to manage pain, may encourage the spread of cancer, according to US researchers.

Speaking at the American Association for Cancer Research in Boston scientists said the opiate promotes the growth of new blood vessels that deliver oxygen and nutrients to tumours.

The researchers said, however, that Progenics Pharmaceuticals’ drug Relistor, used to treat constipation caused by morphine and other pain drugs, appeared to reverse some of the tumour-causing effects in mice and lung cancer cells.

Cancer Research UK said more evidence was needed before a change in treatment was put in place, according to the BBC.