Scientists have developed a natural alternative to morphine that appears to be as effective at killing pain, but has fewer side effects.
Morphine can produce bad side effects
Although morphine is a very effective painkiller, it is addictive, and can cause side effects such as severe constipation, reduced blood pressure and difficulties with breathing.
The new drug is based on proteins called glycosylated enkephalins which are produced by the human body to reduce pain.
It may be particularly useful for the military, which is keen to find drugs that can safely be self-administered by soldiers who are severely wounded during battle.
Our hope is that glycosylated enkephalins can be used to block pain in severe trauma injuries, in victims who could not normally receive narcotics
Lab tests carried out on mice by researchers at the Universities of Arizona and New England have produced highly promising results.
Lead researcher Professor Robin Polt said: "Our hope is that glycosylated enkephalins can be used to block pain in severe trauma injuries, in victims who could not normally receive narcotics."
Other scientists have tried to produce synthetic glycosylated enkephalins.
However, they have never found a way to breach the protective biological membrane that shields the brain from invading toxins, and so have never got the drugs to work.
Professor Polt's team have found that it is possible for enkephalins to cross the blood-brain barrier if they are attached to glucose molecules.
Then, once inside the brain they are able to attach to pain receptors and reduce pain in a way similar to morphine.
Tests on mice showed that the drug produced significantly fewer side effects than morphine, and less signs of addictive behaviour.
They also revealed that the drug works by attaching itself to two different type of pain receptor in the brain, known as "mu" and "delta" receptors.
This makes it more effective than morphine, which only binds to "mu" receptors.
It is also easily broken down by the body into amino acids and sugars, which reduces the risk of toxicity.
The researchers plan further research to test the effectiveness of the drug. It is unlikely to be made available for at least five years.
But Professor Polt believes the work could eventually lead to a whole new class of drugs that may be able to tackle poor memory, attention problems and even depression.
Professor Anthony Dickenson, an expert in neuropharmacology at University College London, told BBC News Online: "There is considerable potential for an opioid-like analgesic that differs from morphine.
"However, only human studies will reveal whether this type of compound has benefits over existing agents.
"For example, subtle side-effects of drugs that may preclude their use in humans [eg hallucinations] may not be revealed in animals."
Details of the research were presented at a meeting of the American Chemical Society.