The exact amount of drugs permissible in your blood while you're behind the wheel were set by government when new drug driving laws were introduced.
-> Gunman shot at ex-girlfriend's car during school run - Leeds Crown CourtWhile possession of any amount of drugs like cannabis or cocaine is illegal, in order to test for the use of drugs while driving, the machines must be calibrated to look for a minimum amount. Those amounts, of course, are very low, but they are set in law.
This is the list of the legal maximum limits for each type of drug tested for.
The police have a 'zero tolerance' approach, which means that having anything above the minimum level is illegal even if you don't know you've taken it.
-> In pictures: Hundreds gather for drugs rally in LeedsThe Gov.Uk guidelines state:
"A zero tolerance approach to 8 drugs most associated with illegal use, with limits set at a level where any claims of accidental exposure can be ruled out
"A road safety risk based approach to 8 drugs most associated with medical uses
"A separate approach to amphetamine that balances its legitimate use for medical purposes against its abuse
"The government is unable to provide any guidance on what amounts of dosage would equate to being over the specified limits.
"There are too many variables, such as physical characteristics, where each person will metabolise the drug at different rates. Eating or drinking will also have an effect on the blood concentration.
‘Illegal’ drugs (‘accidental exposure’ – zero tolerance approach) Threshold limit in microgrammes per litre of blood (µg/L) according to Gov.uk
benzoylecgonine -- 50µg/L
cocaine -- 10 µg/L
delta-9-tetrahydrocannabinol (cannabis) -- 2µg/L
ketamine -- 20µg/L
lysergic acid diethylamide --- 1µg/L
methylamphetamine -- 10µg/L
Methylenedioxymethamphetamine (MDMA) -- 10µg/L
6-monoacetylmorphine (heroin) -- 5µg/L
It is also illegal to drive with certain levels of legal or prescription drugs;
‘Medicinal’ drugs (risk based approach)Threshold limit in blood according to Gov.uk
clonazepam -- 50µg/L
diazepam -- 550µg/L
flunitrazepam -- 300µg/L
lorazepam -- 100µg/L
methadone -- 500µg/L
morphine -- 80µg/L
oxazepam -- 300µg/L
temazepam -- 1,000µg/L
Separate approach (to balance its risk) Threshold limit in blood
amphetamine -- 250µg/L