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Drug driving limits calculator

When it comes to ‘driving under the influence’ many drivers wouldn’t consider their hay fever medication as being particularly dangerous. But some prescription medicine affects your driving and reaction times.

However, the rules are never that straightforward, leaving many confused as to what medication you’re able to take and still be able to drive.

Explore our drug-driving limits calculator to learn more about the legal limits for driving after taking common over-the-counter and prescription medications and legal stimulants.

Louise Thomas, Motor Insurance Expert at Confused.com

Reviewed by Louise Thomas Motor Insurance Expert 2 min read | Published 16/06/2023

Our expert panel reviews all content. Learn more about our editorial standards and how we operate.

Are there prescription drugs that you can't take if you're driving?

The legal limits for driving under the influence of prescription drugs depends on the drug. Our calculator includes some of the more common prescription drugs. Pick one from the list to find out the legal driving limit.

The GOV.uk guidance is that you should talk to your doctor about whether you should drive if you have been prescribed any of these drugs:

amphetamine, for example dexamphetamine or selegiline
clonazepam
diazepam
flunitrazepam
lorazepam
methadone
morphine or opiate and opioid-based drugs, for example codeine, tramadol or fentanyl
oxazepam
temazepam

You are legally allowed to drive after taking these drugs if you’ve been prescribed them, have followed the advice on how to take them by a healthcare professional and they are not causing you to be unfit to drive even if you’re above the specified limits.

What are the penalties for drug driving?

The penalties for driving under the influence of legal drugs can vary but convictions can result in automatic disqualification from driving for 12 months. After being disqualified from driving, you may find fewer and mainly expensive options when looking for car insurance.

If you are convicted of drug driving, you may get:

A minimum 1 year driving ban
An unlimited fine
Up to 6 months in prison
A criminal record

Your driving licence will also be updated to show you've been convicted for drug driving. This will show on your driving licence for 11 years.

It’s illegal to drive in the UK under the influence of any drug that impairs your ability to drive. The rules apply to both legal and illegal drugs. Even if your ability to drive is unaffected, there is legal limit to the amount of some prescription and non-prescription legal drugs allowed in your system.

It may sound a bit strange for there to be an allowable limit for illegal drugs but there is a low allowable driving limit in case of accidental exposure. For example, for the illegal Class A drug cocaine, it’s 10 micrograms per litre of blood.