In order to prosecute under old laws, Police had to prove that an individual’s driving was impaired as a result of drug use. However the newest rules tests for 17 specific drugs and just like with alcohol, it is an offence to drive while under the influence of any of them.
For the most of the UK, the legal drink-drive limit is 80 mg of alcohol per 100ml of blood or 800,000 micrograms per litre. Scotland lowered their drink-drive limit to 50 mg of alcohol per 100 ml of blood in December 2014, which brought them more in line with the majority