Mother was right - if you scratch an itch it will only get worse.
Scratching triggers the release of a nerve chemical that intensifies the maddening sensation, scientists have discovered.
The finding, in mice, suggests the old wives' tale about resisting the urge to scratch an itch has a scientific basis.
Scratching is known to provide temporary irritation relief by generating a small amount of pain. For a short time, nerve cells in the spinal cord carry pain signals to the brain instead of itch signals.
But that is when the trouble starts, researchers have discovered. The brain responds to the pain by releasing the nerve- signalling chemical serotonin, which stimulates more itching.
This result is an itch/scratch/pain/itch vicious cycle.
US lead scientist Professor Zhou-Feng Chen, director of Washington University's Centre for the Study of Itch, said blocking the release of serotonin is not a solution because the neurotransmitter has so many other essential functions.
It is involved in growth, ageing, bone metabolism and mood regulation. Drugs such as Prozac control depression by boosting serotonin levels in the brain.
A more practical answer might be to block the serotonin itch pathway in the spinal cord, said Prof Chen. But until that can be achieved, the best advice is not to scratch.