People who take over the counter medicine and drive could find themselves committing a criminal offence.
New Legislation that came into force on Monday March 2 sets legal limits for how much of a substance - both illegal drugs and prescription medication - motorists can have in their system while driving.
And A new device has been approved that will allow officers at the roadside to test if a motorist has taken cannabis or cocaine.
The DrugWipe system allows officers to determine in less than 10 minutes whether motorists have the illegal substances in their body, using a swab from inside a driver’s cheek.
Those over the limit face a minimum 12-month driving ban; a criminal record, and a fine of up to £5,000, or up to 6 months in prison, or both.
Critics believe many people could be caught out by taking medication such as cough mixture, which contains codeine.
Officers no longer need to prove that a motorist is fit to drive - just that the motorist has an illegal level of drugs in their system while driving.
Any drivers who fail the tests will be arrested and taken to a police station where they will be asked to consent to a sample of their blood being taken. That blood will then be sent away to be examined and if illegal levels of a drug are found, the motorist faces being prosecuted for drug-driving.
If the driver refuses to give consent, they face being prosecuted for the offence of failing to provide a sample of blood for analysis.
The change in the law is designed to make it quicker and easier for drug-drivers to be prosecuted - meaning more of the dangerous motorists can be removed from the streets.
Illegal drugs covered by the new rules include cannabis, cocaine, ecstasy, heroin and ketamine.
Medicinal drugs covered include diazepam, methadone and morphine. The limits for these drugs have been set after advice from a panel of medical experts, at the level where the substances begin to affect driving.
But there is some concern that people could top-up unknowingly by taking medication such as cough syrup.
National statistics suggest one in six drivers who die in crashes in the UK have traces of illegal drugs in their systems - with many of them having taken both drink and drugs.
Studies have shown that motorists who have taken drugs can suffer from slower reaction times, an inability to concentrate, nausea, dizziness, fatigue, erratic and aggressive behaviour, hallucinations, panic attacks, paranoia and tremors.