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 milligram of opiorphin per kilogram of body weight achieved the same painkilling effect as 3 milligrams 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.