Cannabis and Road Traffic: Limits, Detection and Consequences

The legalisation of cannabis under the KCanG has had significant implications for road traffic law. Understanding the THC limit, detection methods, legal consequences and special rules for particular groups is essential for any cannabis user who also drives.

## The New THC Limit: 3.5 ng/ml in Blood Serum

Before the KCanG, Germany applied a 1 ng/ml THC threshold in blood serum – so low that regular users could exceed it days after their last consumption without any actual impairment. The new limit, effective from 22 August 2024, is 3.5 ng/ml, based on an expert commission's finding that this level reflects impairment comparable to a 0.5 per mille blood alcohol concentration.

## Detection and Timing

THC detection windows vary enormously. Occasional users typically fall below 3.5 ng/ml within 3–6 hours of consumption. Regular users may remain above the threshold for 24 hours or longer after their last use. There is no universal "wait X hours" rule. The only reliably safe approach is not driving after consumption.

Roadside saliva tests can detect cannabis use long after blood levels have dropped below the legal limit and serve only as grounds for suspicion, not proof. A blood test is always the legally determinative measure.

## Consequences

Exceeding 3.5 ng/ml results in an administrative offence: a €500 fine, a one-month driving ban and two points in the national driver register. Penalties escalate for repeat offences. Where actual impairment is present alongside an elevated THC level, criminal liability under §§ 315c or 316 StGB may arise, potentially including imprisonment and permanent licence withdrawal.

## Zero Tolerance Groups

Novice drivers in their two-year probationary period and all drivers under 21 face a zero-tolerance rule: any detectable THC level results in consequences, regardless of the 3.5 ng/ml threshold.

## Mixed Consumption

Driving under the combined influence of cannabis and alcohol is treated with particular severity. No additive threshold applies – any combination can result in enhanced penalties, and the pharmacological interaction between the two substances significantly amplifies impairment beyond simple addition.

## Licence Withdrawal and MPU

Serious or repeated violations may trigger referral to the medical-psychological assessment (MPU), which tests whether the driver is physically and psychologically fit to drive safely. Cannabis-related MPUs typically require documented abstinence over several months. The process is costly (€500–€1,000+) and must be passed before the licence is restored.