Scientists have discovered a way to stop post-traumatic stress syndrome in its tracks - by injecting a calming drug into the brain.

Around 30 per cent of people who experience a traumatic event will develop the severe anxiety disorder, which overwhelms a person's ability to cope.

Symptoms can include vivid flashbacks, emotional numbness and nightmares.

While two-thirds recover within a few months some are dogged by the condition for years.

Now researchers from Northwestern University have found the molecular cause of the debilitating syndrome and a way to treat it.

They found that after a traumatic event the brain can become over-stimulated causing an ongoing, frenzied interaction between two brain proteins long after they should have disengaged.

'It's like they keep dancing even after the music stops,' said lead investigator Jelena Radulovic.

However, they found injecting two newly developed research drugs MPEP and MTEP, into the brain's hippocampus ended 'the dance.'

'We were able to stop the development of exaggerated fear with a simple, single drug treatment and found the window of time we have to intervene,' Assistant Professor Radulovic said.

However, the effect only worked if the drug was administered within five hours of the event.

Past studies have tried to treat the extreme fear responses, after they have already developed, she noted.

The study, conducted with mice, was published in the journal Biological Psychiatry.

An exaggerated fear disorder can be triggered by combat, an earthquake, a tsunami, rape or any traumatic psychological or physical event.

'People with this syndrome feel danger in everything that surrounds them,' Ms Radulovic said.

'They are permanently alert and aroused because they expect something bad to happen. They have insomnia; their social and family bonds are severed or strained. They avoid many situations because they are afraid something bad will happen.

'Even the smallest cues that resemble the traumatic event will trigger a full-blown panic attack.'

In a panic attack, a person's heart rate shoots up, they may gasp for breath, sweat profusely and have a feeling of impending death.

Many people bounce back to normal functioning after stressful or dangerous situations have passed. Others may develop an acute stress disorder that goes away after a short period of time. But some go on to develop post-traumatic stress syndrome, which can appear after a time lag.