A team of French researchers say they have found that the human body produces a natural painkiller several times more potent than morphine.
The team from the Pasteur Institute in Paris, France, led by Catherine Rougeot, say that their findings could lead to new pain treatments.
The researchers apparently isolated the chemical opiorphin found in human saliva, and injected it into rats, who had either chemically-induced chronic pain or mechanically-induced acute pain in order to study its pain-beating effects.
The substance was so effective that the rats needed six times as much morphine as opiorphin in order to render them oblivious to the pain of standing on needle points.
The researchers suspect opiorphin may be blocking chemicals called enkephalins from being destroyed.
Enkephalins are found in the central nervous system, and modify the body's response to pain.
The researchers are hoping to identify the conditions that trigger the release of the chemical, and to explore the pharmacological profile of opiorphin so they can study its toxicological effects.
But other scientists appear to be unsure of the significance of the work, and say there is no strong evidence that such chemicals play a role in the physiological control of pain perception.
The research is published in the Proceedings of the National Academy of Sciences.