THC driving laws UK

For THC driving prosecution does not have to prove that a person’s driving was impaired or a level of intoxication was reached, simply that the drugs were present in the system.

THC:
Having more than 2µg of Delta-9-tetrahydrocannibinol, or THC driving, per 100ml of blood is considered a positive test, and is likely to lead to prosecution.
A swab of saliva or sweat at the roadside, and if a positive test gets recorded then the driver will be taken to the police station for an evidential test.
The evidential test will consist of a blood sample being taken for analysis.

• Minimum penalty for THC driving is a 12-month disqualification from driving.
• Maximum £5,000 fine and up to six months in prison.
• More serious charges may also be brought if you were found to have caused an accident, caused injury, or caused damage to property AND tested positive for cannabis.

There is much debate over how much cannabis will put someone in excess of the prescribed limit, as well as how long it will remain in the system for the purpose of saliva testing.
Most groups agree that any intentional use is likely to mean that a person is over the limit for at least a period of 10 hours, in the case of saliva roadside testing.
Heavy users are more likely to be over the limit for a longer period of time.
It is worth noting that, as with alcohol consumption, cannabis is metabolised at different rates in different people, so it is impossible to accurately make any predictions.

Any level of cannabis use is illegal, and the safest option is to avoid its consumption altogether.

There are very few people that have a legitimate, medicinal prescription for cannabis,
If you test positive at the roadside, you should have a medical defence that you can call on in court as long as you can produce a valid prescription.

Cannabis is detectable by saliva testing

Just like blood testing, saliva testing detects the presence of parent drugs and not their inactive metabolites. This results in a shorter window of detection for cannabis by saliva testing. Delta 9 THC is the parent compound.

If saliva samples are tested in a lab:

• The detection level can be as low as 0.5 ng/mL (up to 72 hours after intake) and if an onsite instant saliva drug test is used, the cut off level is generally 50 ng/mL
  (up to 12 hours after intake).

Further:
Eight prescription drugs have been included in the list of drugs to be tested by police, with limits set above typically prescribed levels. As well as these prescription drugs, the list also includes eight illegal narcotics, notably cocaine and cannabis which are known to impair reactions and functions.