A roadside swab test, introduced in March 2015, can catch drug offenders on the roads.
Under the Road Traffic Act 1988, police must prove someone's driving is impaired due to drugs in order to prosecute. But the latest rules specified limits for 17 drugs, similar to what we have with alcohol, making it an offence to drive after taking them.
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. However if a driver is affected in any way - passively or otherwise - they are committing an offence.
So what are the rules when it comes to drugs and driving?
1. Benzoylecgonine (cocaine) - 50 micrograms per litre of blood (µg/L)
2. Cocaine - 10µg/L
3. Delta-9-Tetrahydrocannabinol (cannabis and cannabinol) - 2µg/L
4. Ketamine - 20µg/L
5. LSD - 1µg/L
6. Methylamphetamine - 10µg/L
7. MDMA (ecstasy) - 10µg/L
8. Heroin and diamorphine - 5µg/L
1. Clonazepam (used to treat seizures and panic disorder) - 50µg/L
2. Diazepam (anti-anxiety) - 550µg/L
3. Flunitrazepam (Rohypnol - sedative) - 300µg/L
4. Lorazepam (anti-anxiety) - 100µg/L
5. Methadone (heroin substitute) - 500µg/L
6. Morphine (pain relief) - 80µg/L
7. Oxazepam (anti-anxiety) - 300µg/L
8. Temazepam (anti-anxiety and sedative) - 1,000µg/L
9. Amphetamines (eg dexamphetamine used for conditions such as ADHD) - 250µg/L
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; this legislation gives police the power to test and arrest motorists who are suspected of driving over the new levels.
Unlike the existing 'impairment' offence, this 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 drive and take prescription medicine, it may be helpful to keep evidence of this with you in case you're stopped by police.
If you're convicted of drug-driving you'll get:
• A minimum one-year driving ban
• A fine up to £5,000
• A criminal record
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.
Police forces were given an additional £1million to train officers, purchase drug screening equipment and pay for samples to be analysed.
In the first year after the new law's intorduction in 2015, nearly 8,000 arrests were made in England and Wales. Stats from 35 of 43 forces showed 7,796 arrests were made between March 2015 and April 2016.