Police are warning motorists that THC, the active ingredient in cannabis, can show up in a person’s system as much as three weeks after it was consumed. Any motorist that tests positive for cannabis runs the very real risk of losing their driving licence and paying a hefty fine.

"It is important that we warn people," north Zealand police spokesperson Henrik Suhr, told the Helsingør Dagblad.  “Marijuana can still be tested for three weeks later, and we have a zero-tolerance policy. Drivers should not drive for three weeks after they smoke.”

The consequences for even a first time offender caught with THC in their bloodstream are serious: a three-year driving ban and a fine equivalent to one month of the driver’s net salary. As little as 0.001 milligrams of THC per kilogram of blood is enough to incur the penalties, which, in practice, amounts to a zero-tolerance policy.

The strict policy has its share of detractors, especially among those who fear that someone who did not smoke cannabis themselves but simply inhaled passive smoke from someone nearby could wind up losing their right to drive. 

Costly Sunday drive
The Copenhagen police department has scores of complaints on file from people who have lost their licence who said that they were well over the effects of smoking long before they sat behind the wheel. One man lost his licence and his job after he was found to have THC in his blood on the Sunday after he smoked one joint the Wednesday before; more than three days earlier.

READ MORE: Experts: Zero-tolerance on ‘cannabis driving too severe

While some in parliament would like to see the rules relaxed, police warn that the current laws are still in place and advise drivers to wait three full weeks after lighting up before they hit the road.