The potential implications of this finding are really incredible

Professor Jon Levine, University of California, San Francisco

However, recent studies have suggested the drug's full potential as an analgesic has not been tapped.

Now that potential appears to have been fully realised in research by a team from the University of California, San Francisco (UCSF).

Lead researcher Professor Jon Levine, director of the NIH Pain Center at UCSF, said: "The potential implications of this finding are really incredible."

Gender difference

Previous research by the UCSF team on 48 patients following jaw surgery found that kappa-opioids provided substantial pain relief for women, but not for men.

This helped to explain why the drugs had previously had a poor reputation, as until the early 1990s only men were included in most clinical trials.

Then in 1999 the researchers discovered that a low dose of one form of kappa-opioid, known as nalbuphine, actually causes a marked increase in pain in men.

If somebody is in severe pain you need to treat them with a drug you know is going to relieve it and morphine is so strong

Professor John Henry, St Mary's Hospital, London

Higher doses, however, did have a painkilling effect.

In women, low doses had no effect, and higher doses produced a stronger and longer lasting painkilling effect than was seen in men.

In their latest study, the researchers combined a low dose of nalbuphine with another drug naloxone. This blocked the increase in pain in men, and produced very effective, prolonged pain relief in patients of both sexes.

Opioids, which also include heroin, are synthetic versions of drugs originally derived from the juice of the opium poppy plant.

They work by blocking receptors on cells that normally receive and transmit pain signals.

Scientists believe that kappa-opioids may block pain at some type of receptors, and enhance it at other types.

However, naloxone may modify the reaction so that pain is blocked at all types of receptor.

Professor John Henry, head of the department of academic emergency medicine at St Mary's Hospital, west London, said a lot more research was needed to compare the drug cocktail with other analgesics, and for various levels of pain.

He told BBC News Online: "If somebody is in severe pain you need to treat them with a drug you know is going to relieve it and morphine is so strong.

"We could be a long, long way from this drug being in the ward trolley."