A new study has revealed that common pain relief medication such as morphine can actually encourage the growth and spread of cancer cells.

Opiate-based painkillers have been shown to stimulate cancer growth. Two new studies have shown how shielding lung cancer cells from opiates reduce cell proliferation, invasion and migration in both cell-culture and mouse models.

The researchers focussed on the mu opiate receptor, where morphine works, as a potential therapeutic target.

"If confirmed clinically, this could change how we do surgical anesthesia for our cancer patients," said Dr Patrick A. Singleton, assistant professor of medicine at the University of Chicago Medical Center and principal author of both studies.

"It also suggests potential new applications for this novel class of drugs which should be explored," he added.

A 2002 palliative-care trial showed that patients who received spinal rather than systemic pain relief survived longer. Soon after that, Singleton's colleague, anesthesiologist Jonathan Moss, noticed that several cancer patients receiving a selective opiate blocker in a compassionate-use protocol lived longer than expected.

Moss's palliative-care patients were taking methylnaltrexone (MNTX), developed in the 1980s for opiate-induced constipation by the late University of Chicago pharmacologist Leon Goldberg. He modified an established drug that blocks morphine so that it could no longer cross the protective barrier that surrounds the brain.

So MNTX blocks morphine's peripheral side effects but does not interfere with its effect on pain, which is centered in the brain.