THC levels in the blood and driving: what the law says

Blood alcohol rests on a dose-effect relationship documented for decades: the higher the level of alcohol in the blood, the more predictable the motor and cognitive impairment. THC follows very different kinetics. Its blood concentration falls quickly after consumption, whereas certain subjective effects can persist longer, particularly in a regular user in whom THC remains stored in fatty tissue and is released gradually.

Work in road toxicology converges on one finding: to date there is no blood or saliva threshold of THC universally correlated with a level of impairment, unlike the 0.5 g/l benchmark for alcohol. The same THC level can correspond to very different degrees of impairment depending on frequency of consumption, individual tolerance and the time since the last dose. It is this scientific uncertainty that fuels the debate, in France as in other countries, about how appropriate the currently applied thresholds are.

Article L235-1 of the Code de la route penalises driving after using narcotics as soon as a saliva or blood test reveals the presence of THC, with no tolerance threshold tied to a level of impairment. The Cour de cassation, in its ruling of 21 June 2023, confirmed that the commercial legality of a cannabinoid-based product in no way exempts the driver: only the test result counts.

A CBD product compliant with French law contains up to 0.30 % THC. That trace, tiny though it is, can nonetheless trigger a positive saliva test, particularly after recent or repeated consumption. THC is detectable in saliva for several hours after occasional use, and longer after regular use. Before getting behind the wheel, it is better to opt for a CBD isolate certified free of THC, whose certificate of analysis (CoA) confirms the absence of the molecule.

Setting a legal threshold based on actual impairment would presuppose a scientific consensus that does not yet exist for THC. In the absence of any such reliable benchmark, the French legislature has chosen caution: the mere presence of THC, whatever its concentration, suffices to establish the offence. That choice protects road safety, but it does not distinguish a genuinely impaired driver from a consumer of legal CBD who has taken in a minute trace of THC.

Does a high level of THC in the blood mean stronger impairment?

Not necessarily. Research in toxicology has not established a reliable correlation between blood THC concentration and the actual degree of impairment, unlike alcohol.

Can you be penalised while using a legal CBD product?

Yes. The commercial legality of a CBD product does not stop a saliva test positive for THC from constituting an offence, according to the case law of the Cour de cassation.

How long does THC remain detectable after consuming full-spectrum CBD?

That varies with frequency of use and with the product, generally several hours after occasional use and longer after regular use. An isolate certified free of THC reduces that risk.

What should you do before driving if you use CBD?

Check the product's certificate of analysis, favour an isolate free of THC, and avoid any consumption in the hours before driving, whatever product you use.