Police can already prosecute motorists caught driving under the influence of drugs, including medicinal drugs. But the new rules, which are set to come into force next month (2nd March 2015), introduce specified limits for 16 drugs, similar to what we have with alcohol.
The Government decided against a zero limit 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 drink-drive limit for the majority of the UK is 80 milligrams of alcohol per 100ml of blood - the equivalent of 800,000 micrograms per litre. In December 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 existing 'impairment' offence, the new law provides a medical defence if you're taking a prescription 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.