Drug-driving rules were updated in 2015 to include a list of non-illegal drugs which have specified drive limits, similar to what we have for alcohol.
At the time, the Government decided against a zero tolerance approach to the new additions as certain medicinal drugs can be absorbed in the body and produce trace effects.
They also didn't want to risk penalising drivers for accidental exposure to drugs, such as inhaling cannabis smoke in a public place.
So, what's on the list?
The legal alcohol units for driving for the majority of the UK is 80 milligrams of alcohol per 100ml of blood - the equivalent of 800,000 micrograms per litre. In 2014, Scotland’s drink-drive limit was lowered to 50 milligrams of alcohol per 100ml of blood; bringing them in line with most other European countries.
The limits for the majority of prescription drugs are above the normal doses; the new legislation will give police the power to test and arrest motorists who are suspected of driving over the new levels.
Unlike the old 'impairment' offence, the law now provides a medical defence if you're taking a prescription medication in accordance with medical instructions provided, of course, you're not impaired.
If you're convicted of drug-driving you'll get:
Your driving licence will also show a conviction for drug-driving and it will stay on there for 11 years.
A conviction for drug-driving also means you may not be able to get car insurance; Admiral and its sister companies will not cover anyone who has been found guilty of a drug-driving offence.
If you have a driving job your employer will see the conviction on your licence and you may have trouble travelling to certain countries, such as the USA.