Saliva from humans has yielded a natural painkiller up to six times more powerful than morphine, researchers say.

The substance, dubbed opiorphin, may spawn a new generation of natural painkillers that relieve pain as well as morphine but without the addictive and psychological side effects of the traditional drug.

When the researchers injected a pain-inducing chemical into rats’ paws, 1 gram of opiorphin per kilogram of body weight achieved the same painkilling effect as 3 grams of morphine.

The substance was so successful at blocking pain that, in a test involving a platform of upended pins, the rats needed six times as much morphine as opiorphin to render them oblivious to the pain of standing on the needle points.

“Its pain-suppressive effect is like that of morphine,” says Catherine Rougeot at the Pasteur Institute in Paris, France, who led the research. “But we have to test its side effects as it is not a pure painkiller,” she says. “It may also be an anti-depressive molecule.”