Is it illegal?
Yes it is! If you are under the influence of drink or drugs you are classed as unfit to drive and are therefore guilty of an offence
If the police stop you and suspect you may be using drugs they can do something called a ‘Field Impairment Assessment’. This assessment includes a number of checks like asking you to walk in a straight line and checking-out the size of your pupils. Once they’ve done these tests if they still think you’re unfit to drive they will arrest you and will have to undergo a blood test at a police station. If the blood test is positive for drugs then you could be charged with committing an offence.
It is important to remember that being unfit to drive can also be a result of taking medicines and prescribed drugs not just illegal drugs. Always check with your pharmacist if you take medications and have to drive.