Police have launched a campaign to crack down on drug-driving.
The summer initiative aims to hammer home the fact that driving under the influence of drugs - both prescription and illegal substances - can be as dangerous as drink-driving.
If caught, drivers face the same punishment as those over the alcohol limit: a minimum one-year driving ban and fine of up to £5000.
Chief superintendent David Snelling, of London's Metropolitan Police, said: 'The chances of being involved in a collision are significantly increased, which may result in serious injury or death.
'As well as these dangers, you may lose your job or end up in prison.'
Drug-drivers harder to identify
In order to arrest a driver for drug-driving, police must prove the suspect is unfit to drive through the use of illegal or prescription drugs.
Suspects will typically be asked to take a roadside test, which may include examining the pupils of their eyes, standing on one leg and walking in a straight line.
Identifying drug-drivers is more complex than catching drink-drivers, however.
There is no testing device available for all types of drugs, although the Home Office is working on a machine that could be available by the end of the year.
While traces of so-called softer drugs such as cannabis often remain in the bloodstream for several days, making it hard to prove someone was driving under its influence, harder drugs can pass through someone's body much more quickly.
Prescription drugs, such as anti-depressants, painkillers, antihistamines and cough mixtures can all slow a person's reactions, too, and police are warning that anyone taking them should carefully follow medical advice on driving afterwards.
The summer's drug- and drink-driving campaign will run until September 2.
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