It's the old wives tale that our mothers have been telling us for decades: if you scratch that itch, it'll only get worse.
According to a new health study this could actually be true, as scratching triggers the release of a nerve chemical that intensifies the maddening sensation.
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's 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 cycle.
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"The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain," said Professor Zhou-Feng Chen, director of Washington University's Centre for the Study of Itch.
"But as serotonin spreads from the brain into the spinal cord, we found the chemical can 'jump the tracks', moving from pain-sensing neurons to nerve cells that influence itch intensity.
"This fits very well with the idea that itch and pain signals are transmitted through different but related pathways. Scratching can relieve itch by creating minor pain. But when the body responds to pain signals, that response actually can make itching worse."
In the lab experiments, Professor Chen's team injected mice with a substance that caused them to itch and scratch.
Tests revealed the role played by a serotonin-sensitive molecular receptor known as 5HT1A, found on itch-specific GRPR neurons in the spinal cord.
Stimulating the receptor was like turning on an "itch switch".
Prof Chen, whose research appears in the journal Neuron, said: "We always have wondered why this vicious itch-pain cycle occurs. Our findings suggest that the events happen in this order.
"First, you scratch, and that causes a sensation of pain. Then you make more serotonin to control the pain. But serotonin does more than only inhibit pain. Our new finding shows that it also makes itch worse by activating GRPR neurons through 5HT1A receptors."
Blocking the release of serotonin is not a solution because the neurotransmitter has so many other essential functions, said the professor.
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 that itch.