A ruling by Finland’s Supreme Court means that motorists will likely not be charged for driving under the influence days after using cannabis.

The influence of cannabis on users dissipates after some hours, however its "fingerprints" remain in the body for even longer. The court ruling aims to recognise that compounds related to metabolising cannabis may remain in the body for days after using the drug, although the user's ability to drive may not be impaired.

In Finland using cannabis is still illegal and drivers are still likely to be charged for the offence if they get behind the wheel hours after indulging in its use.

Cannabis usage leaves a substance known as tetrahydrocannabinol or THC in the body, prompting the production of a metabolite, carboxytetrahydrocannabinol (THC-COOH, where a metabolite is a substance required for or produced from metabolism.

THC is a psychoactive substance, which intoxicates people, so detecting it in a driver’s bold always results in a charge for driving under the influence. In Finland, police generally also issue a DUI citation if they find the metabolite THC-COOH in the blood.

However there is a major difference between the compounds.

Depending on the dosage taken, THC is known to leave the body between six and eight hours after cannabis use. However metabolites like THC-COOH may show up in blood samples for days or weeks longer. It is the equivalent of a cannabis "fingerprint" that reveals use of the drug in the recent past.

Unlike THC, the metabolite THC-COOH does not have an intoxicating effect and therefore does not impair the ability to drive.

According to Teemu Gunnar, a forensic toxicologist with the National Institute of Health and Welfare, THL, it makes sense that identifying THC-COOH in the blood should not automatically lead to a DUI sanction.

"It’s quite rational that finding it several days after use would not lead to a drunk driving penalty because it does not affect the ability to drive," he said.

Supreme Court decision changes police and prosecutor practices

A few years ago the Supreme Court dismissed an appeal court conviction in the case of a driver who had smoked cannabis before getting behind the wheel. THC-COOH "does not affect the ability to drive nor endanger traffic safety even at high levels," the court said in its ruling.

Since that ruling, police and prosecutors have changed their practices to bring them in line with the court’s judgment. Prosecutors no longer press charges solely on the basis of the presence of THC-COOH in the blood, or when the police have confirmed that there is no reason to suspect an offence was committed.

"We have acted in accordance with the Supreme Court guideline, in other words for the most part we do not lay charges. Before [the precedent] it led to charges," noted district prosecutor Tero Kekki.

"We have reacted and practices have changed. So we don’t do any unnecessary work," commented detective inspector Ilpo Pajunen of the Helsinki police department.

In spite of the precedent-setting case, officials have still sanctioned motorists for driving days after cannabis use in the period since the ruling.

The reason for the lag can be found in Finland’s criminal code, which calls for a driver to be cited for DUI if their blood has "an active substance or metabolite of a drug used" during the time they were driving or shortly afterwards.

In its judgment, the Supreme Court noted the need for legislative changes. Preliminary drafts state that the drunk driving offence should not be applied to cases where a motorist’s ability to drive cannot be said to have deteriorated due to drug use.

Increase drug-related in DUIs

Growing numbers of drivers on Finnish roads are people pulled over for DUIs caused by drug use, with amphetamines the substance most commonly detected in these cases. Last year, police reported 5,107 such cases.

Cannabis metabolites were the second-most-frequently found substances. In 2018, records showed 1,784 hits of THC in lab samples and 3,794 of THC-COOH.

Moreover, the majority of blood samples handed over to the National Bureau of Investigation, NBI, contained several intoxicants. From the perspective of the subjects’ ability to drive, the least significant substance is not necessarily THC, but other possible drugs – or even the more traditional alcohol.

Police also said that drivers are sent for drug testing when they feel there is reason to suspect drugs are involved. For example, when the breathalyser registers no alcohol, but driving ability is clearly impaired.

Pajunen said that he suspects that drivers who have used cannabis days before and are then able to manage a vehicle do not end up being tested for drugs.

"There must be some kind of trigger to suspect the influence of drugs. Behaviour, driving mistakes or something like that," he added.