Strongly illegal: Driving with THC in the blood


A single number from a measuring device decides whether someone has committed an offence, even if nothing else has happened and even if no abnormalities are found in the medical examination: in other words, the suspect is almost certainly fit to drive.


Difficult subject! Actually, you have to decide: either smoke pot or drive a car. And we don't mean abstain for one night and then hope it will be okay. No, it's either drive and never smoke pot or smoke pot and never drive. Everything else is very risky.


The penalty is high because it's a misdemeanor. Misdemeanor is a misdemeanour. There is a fine, plus fees and investigation costs (together quickly over 2,000 francs), plus a criminal record entry, withdrawal of the “ticket” for at least three months and clarification of the fitness to drive by the road traffic office.

Strongly illegal: Driving with THC in the blood

Specific examination or blood work?

In the past, a doctor had to carry out a specific examination to check whether or not a motorist was fit to drive. But since the introduction of zero tolerance in 2005, the detection of THC in the blood is sufficient to prove driving incapacity.

The regulation states that “Tetrahydrocannabinol (cannabis)” can be detected in the blood and cause impaired driving. This constitutes a misdemeanour (highly illegal) and is punished accordingly. In addition, there is a temporary withdrawal of identification and then the procedure at the traffic office.

Detection in the blood

If the blood is examined using forensic methods, the amount of THC present in it can be determined. It is indicated in nanograms per millilitre of blood (ng/ml) or micrograms per litre of blood (µg/l). (Similar with alcohol: the alcohol per mille content can be determined in the blood). But what does the active substance content in the blood tell us? Can these values be used to determine whether a person is actually stoned or alcoholic? In the case of alcohol, the legislator says yes and means that more than 0.5 per mille is not permissible for driving a vehicle. Conversely, this means that someone with 0.4 per mille alcohol may drive a car.

In the case of illicit drugs, however, the limit is supposed to be zero, or the limit of what the measuring device can just measure (about one microgram per litre). With the ordinance introduced in 2005, the presence of an extremely small amount of THC in the blood is therefore sufficient to establish the inability to drive: the detection limit is 1.5 µg/l. The average value of THC in the blood is then reduced by 30 %. A 30 % deduction is made from the average THC value measured. A measured blood value of 2.2 is then sufficient for the detection of THC in the blood and thus the inability to drive is proven, even if no abnormalities can be detected in the driver.

This is an extremely low value that has nothing to do with a concrete stoned state / impairment. Because shortly after consumption, values of dozens to over 100 micrograms per liter of blood can be measured.

How long after consumption does the THC level in the blood remain above 1.5 µg/l?

This varies greatly from individual to individual and according to the amount consumed. Someone who only “smokes one” every few weeks may be below this value after six hours. But those who consume daily may always have a higher value. This would only fall below 1.5 after several days of abstinence.

What does this mean for THC users?

A person who occasionally smokes pot should not have smoked pot for at least six hours before driving. The one who smokes pot regularly, on the other hand, is never considered fit to drive, unless he / she would skip three days of smoking pot before driving. Here we can clearly see that zero tolerance goes far beyond the goal of removing only those who are unfit to drive. It makes legal driving impossible for people who regularly consume THC, even if they always put a night between consumption and driving!

Traffic stop

Occasions for police traffic stops:

  • a real error in driving a motor vehicle (e.g. crossing a safety line, driving too fast, a defective light or being involved in an accident)
  • driving slowly, e.g. because someone is unfamiliar with the location
  • a large-scale traffic control without specific reason
  • Targeted search for people who have previously come to attention with cannabis.

Once a police check is underway, the smell of cannabis, reddened eyes or an entry for cannabis in the police database or criminal record can lead to further investigations: Search of the car, the driver, the passenger……

A rapid drug test may actually only be carried out if there is a concrete suspicion of impaired driving ability: smoking pot behind the wheel, red eyes, unsafe driving, unclear pronunciation, carrying cannabis, etc. In fact, however, the police have enormous leeway here.

Some police forces still carry out rapid tests, which normally detect THC-COOH in saliva or urine. If such a test is positive, a medical examination and a THC blood test in hospital can be ordered. This is because THC-COOH is not legally relevant for driving under the influence of drugs. The only thing that counts here is the THC level in the blood.

Other police forces have introduced new test procedures that do not require the (inaccurate) rapid drug tests. The driver is questioned by the police in a standardised way for three minutes and, on the basis of the answers and visible elements (salivation, eye behaviour, etc.), they decide whether they want to take the person to hospital for more detailed clarification. If they want to do so, the police must:

  • question the suspects and draw up a protocol
  • chauffeur her to the hospital for the blood test.
  • suspend a driver's licence
  • report the suspects to the public prosecutor's office
  • Report to the Road Traffic Office.

Medical examination in hospital

In hospital, the persons concerned are examined by a doctor and the result is recorded. However, this concrete examination is of little value in the case of drug use. Even if the result (as is very often the case) is “no abnormalities” this is of no use to the person concerned: only the result of the blood test is decisive. (If the medical examination finds abnormalities, however, there may be further problems). It can take weeks or months for the blood to be tested. The driver's licence remains confiscated or is returned with reservations.

Three substances are measured in the blood:

  1. THC (tetrahydrocannabinol)
  2. OH-THC (hydroxy-THC)
  3. THC-COOH (THC carboxylic acid)

The THC value is decisive for the concrete driving ability. OH-THC is not relevant (so far). The THC-COOH value is relevant for the general fitness to drive, it gives information about the consumed amount.

All three substances are measured in micrograms per liter of blood (µg/l). High values are for example 23-7-210, low 2-1-30.

Charge sheet

The police report, the results of the medical examination and the results of the blood test are sent to the public prosecutor's office. The public prosecutor's office then issues a penalty order on the basis of these results. There are two different variants:

The THC level in the blood is under 1.5 micrograms per liter of blood (or 2.2 incl. safety margin):
⇒ Penalty order for consumption

THC is detected in the blood, but below the amount relevant for traffic law. Nevertheless, this proves illegal THC consumption and this is therefore punished (fine).

The costs of the investigation (over 1,000 francs) are charged to the accused, so that the bill comes to over 2,000 francs.

Although this is only permissible if a concrete suspicion has been provided, there is a tendency to simply try to impose the costs on those affected.

⇒ The THC level in the blood is at least 1.5 micrograms per liter of blood (or 2.2 incl. safety margin):
⇒ Penalty order for driving under the influence of drugs

Here, the illegal THC consumption is only the smaller part of the punishment (violation), really illegal is the driving under drugs (misdemeanor). This is legally proven with such a value.

Now, in addition to a fine for consumption, there is also a fine (10 to 40 daily rates) for driving under the influence of drugs, and the costs of the hospital must also be paid. The driver's license is revoked for three months. There is an entry in the criminal record, so you have a criminal record.

Police check on non-car drivers

A classic case is THC use outside, where someone is caught by the police and confesses to daily use in the interview, for example.

A hefty party or a loud argument at home can lead to the police stopping by and so seeing pot paraphernalia. Then a protocol is taken and also asked about the frequency of consumption.

Or a cyclist or pedestrian is involved in an accident, for example at a pedestrian crossing. In such cases, blood samples are taken from all those involved and questioning is also carried out.

Depending on the incident, the police report the person concerned for consumption, disturbance of the peace, etc. and file a report with the Road Traffic Office, even if the incident had nothing to do with motorised road traffic.

Measures taken by the Road Traffic Office

If the Road Traffic Authority is informed for any reason (e.g. by the police) that a person with a driving licence has used THC, it checks whether the person's cannabis use poses a risk to road traffic. If the answer is in the affirmative (e.g. because the person concerned has been found to use cannabis at the wheel or to use it regularly), a precautionary suspension may be imposed, i.e. the driver's licence may be withdrawn for an indefinite period.

⇒ If it is only a confessed consumption of one or two times per week and this is in no way connected with road traffic, the STVA will normally not take any action. The same applies if the THC-COOH content is below 40 µg/l (if the result of a blood test is available). The matter is then closed (but remains on file).

⇒ If THC of 1.5 µg/l or more has been legally detected at the wheel, a fitness to drive assessment will be initiated. Likewise, if the THC is below 1.5, but consumption of more than twice a week is reported and also if the THC-COOH value is above 40. Carrying cannabis also raises doubts about fitness to drive (no separation of consumption and road traffic). This all then runs under “suspicion of drug addiction”.

Traffic medical report

A urine test, blood test and traffic medical report (costs over 1,500 francs) should clarify whether drug addiction is present. The expert opinion then recommends whether the person examined can be granted fitness to drive and whether there should be conditions for this, i.e. fitness to drive:

  • can be endorsed without conditions
  • can be approved with conditions
  • should be rejected.

Based on this opinion, the decision of the Road Traffic Office is made. The driver's license is issued depending on:

  • unconditionally redelivered
  • re-issued with conditions (depending on the case, these vary from urine tests over half a year, a whole year or two years; possibly with further traffic medical reports every 6 or 12 months)
  • not issued again (precautionary withdrawal / precautionary withdrawal). First, a further three or six months of THC abstinence must then be documented before a possible granting of the driving licence is discussed - then also with the described conditions.

Abstinence Control

A urine check in hospital costs over 200 francs and usually has to be dispensed once a month. This results in costs of around 3,000 francs per year.

Ultimately, the authorities simply want to see total abstinence if the license is to remain: negative urine controls; clear demarcation from previous consumption; insight that life is better without consumption (with concrete examples); that one has contact with other, abstinent people and no longer with THC users. Increasingly, they also want to know whether there is a shift to alcohol consumption (hair samples are then requested for this purpose).

If all conditions have been fulfilled and the fitness to drive is unreservedly endorsed, the driving licence is returned without conditions. Otherwise, the mills of the road traffic office grind on.

THC use or driver's license

Since urine samples remain positive for a very long time after consumption and the limit is reached quickly, those affected really need to think about this: Driver's license or THC consumption? Those who want or need to keep “the ticket” can no longer continue to consume THC. Even legal hemp products without psychoactive effects may no longer be consumed, as they also lead to positive urine samples can lead to positive urine tests (see Food and CBD Hemp).

The "THCs"

Three levels are determined in the blood:
THC: active substance, legally relevant. Driving under the influence of drugs is considered proven from 1.5 µg/l.
OH-THC: active substance, not legally relevant.
THC-COOH: degradation product, legally indirectly relevant, because more than 40 µg/l is considered as an indication of a possible drug addiction.

Road Traffic Office

The Road Traffic Office is responsible for clarifying the general fitness to drive, i.e. whether a person may still have a driving licence at all (drug addiction rules this out). It arranges for expert reports to be drawn up and decides on measures to be taken (withdrawal by way of precaution, controlled abstinence).

Drug addiction?

The consensus opinion in forensic medicine is that a consumption of once to twice a week is still just tolerable (occasional consumption). From three times a week onwards, they assume “no longer occasional/frequent use”. “no longer occasional/frequent use”… which should justify a suspicion of drug addiction.

Prosecutor's Office

The public prosecutor's office conducts the criminal proceedings. For the punishment it is only relevant whether someone drove a car with at least 1.5 µg/l THC in the blood. If this is the case, the public prosecutor's office issues a penalty order for driving under the influence of drugs (which is a misdemeanour).

Penalties for driving under THC

A federal court ruling puts certain points into perspective.
However, this still needs to be clarified in more detail, because most cases end with a conviction.

Discontinuation orders

Repeat offence during the probationary period

The first time you are caught driving under the influence of drugs, the penalty is usually conditional, so you don't have to pay the costs for the daily fines (but you do have to pay the fine, fees and other costs). But if you get caught again during your probation, it can quickly become enormously expensive. In this case, the prosecutor can revoke the old sentence and form a new sentence from the old and the new case.

Example of an unconditional penalty

Overview and summaries

Specific cases Driving licence

Leaflets and directives

Report on THC limits (2020)

An interesting report from the Institute of Forensic Medicine of the University of Basel on the subject of THC in the blood and driving ability/ability to drive. However, we would like to point out: This is a discussion paper and not yet implemented in current law!

Fact sheet THC limits (short version)

Report THC limits (45 pages)

Summer 2021: New THC limit in road traffic?

Institute of Forensic Medicine Zurich:

Swiss Society of Forensic Medicine SGRM:

Federal Roads Office FEDRO:

Road Traffic Act and Ordinances

Traffic Regulations Ordinance

Traffic Licensing Ordinance

en/thc_recht/autofahren.txt · Last modified: 2021/09/06 16:19 by sos
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