EXPLAINED: Why Scratching Makes You Itch More

We all know that a good scratch will get rid of an itch – only for it to come back with a vengeance.

Now scientists claim to have finally worked out exactly why scratching always makes us itch even more.

Research indicates that scratching causes the brain to release serotonin, which intensifies the itch sensation.

The findings, in mice, indicate the same vicious cycle of itching and scratching is thought to occur in humans.

And the research also provides new clues that may finally help break that cycle – particularly in people who experience chronic itching.

Scientists have known for decades that scratching creates a mild amount of pain in the skin.

Senior investigator Doctor Zhou-Feng Chen, director of Washington University’s Centre for the Study of Itch, said that pain can interfere with itching – at least temporarily – by getting nerve cells in the spinal cord to carry pain signals to the brain instead of itch signals.

Dr Chen said: ‘The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain.’

As serotonin spreads from the brain into the spinal cord, the chemical can ‘jump the tracks’ – moving from neurons that sense pain to nerve cells that influence the intensity of an itch, she added.

‘Scratching can relieve itch by creating minor pain. But when the body responds to pain signals, that response actually can make itching worse.’

Serotonin is involved in growth, ageing, bone metabolism and in regulating mood.

Dr Chen added: ‘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 findings shows that it also makes the itch worse.

The research was published in the journal Neuron.