The human body produces a natural painkiller several times more potent than morphine, research suggests.
When given to rats, the chemical, called opiorphin, was able to curb pain at much lower concentration than the powerful painkiller morphine.
The French team said their findings could be lead to new pain treatments.
But other scientists were unsure of the significance of the work, which is published in the Proceedings of the National Academy of Sciences.
The researchers isolated the chemical in human saliva, although they believe it may also be present in other parts of the body.
To study its pain-beating effects, they injected opiorphin into rats, who had either chemically-induced chronic pain or mechanically-induced acute pain.
The researchers found injections of one milligram per kilogram (mg/kg) of opiorphin could suppress the rats' pain to the same extent as injections of 6mg/kg of morphine, which is used to manage severe pain.
The researchers are not yet certain of the exact mechanism, but they believe opiorphin may be stopping enkephalins, chemicals found in the central nervous system that modify the body's response to pain, from being destroyed.
Possible mechanisms
The researchers now hope to identify the conditions that trigger the release of the chemical, and lead researcher Professor Catherine Rougeot, of the Institut Pasteur in Paris, France, said their findings could potentially lead to new treatments for treating pain.
But she cautioned: "First we need to explore the pharmacological profile of opiorphin and to study its toxicological effects."
John Wood, professor of molecular neurobiology, at University College London, said: "The discovery that human saliva contains a pain-killing protein is very interesting, and follows on from the discovery of related protein activities in rats and cows. These proteins all stop the breakdown of natural morphine-like proteins that block pain pathways in the brain.
"However, drugs that act in a similar way have not progressed to the clinic, and there is no strong evidence that these proteins play a role in the physiological control of pain perception. The significance of these findings for pain control is thus still uncertain."
