Earlier tests of the poison found that it stopped pain signals from the nervous system travelling to the brain, but it also blocked other important nerve signals, researcher Mac Christie told the Sydney Morning Herald newspaper on Monday. 

But Christie, director of research at the Pain Management Research Institute at Sydney's Royal North Shore hospital, said certain molecules had now been identified which blocked only the pain signals. 

The cone shell had developed "very selective molecules that have evolved to shut down various aspects of nervous function", Christie said. 

Testing on humans

A few of the 600 species of the fish-eating cone shell are
poisonous enough to kill a human. They inject their poison via a barb that shoots out when they are disturbed. 

Christie said animal studies had shown a pain-relieving
dose would have to be increased one thousand times before side effects begin to appear. 

He hopes to begin testing the potential breakthrough medicines on human volunteers later this year before seeking approval to try them on people with severe pain syndrome. 

He said the potential pain relief would be useful for cancer
patients, severe arthritis sufferers and people with nerve or spinal cord injuries whose pain could not be relieved by morphine.