How long does THC stay in your system? Blood, urine, saliva and hair

How long does THC stay in your system? The detection windows at a glance

Blood has the shortest detection windows, hair the longest. Saliva and urine lie in between. The values in the table come from studies in which use and abstinence were monitored. They are a guide to typical courses, not a guarantee for the individual case.

Occasional here means less than weekly or a few single occasions. Frequent means daily or almost daily over weeks to years. Anyone in between usually lies in between for the time windows, too.

Why does THC stay detectable for so long?

THC stays detectable for a long time because it is highly fat-soluble. After intake it spreads quickly into tissue, especially fatty tissue, and is slowly released back into the blood over days to weeks.

The liver breaks THC down in several steps. First 11-hydroxy-THC is formed, which is itself still psychoactive. This is converted into THC-COOH, a breakdown product without intoxicating effect, which is bound to glucuronic acid and excreted in urine and faeces. Because THC-COOH keeps being supplied from the tissue store, it remains measurable much longer than THC itself.

In a urine study with prison inmates, the excretion half-life of THC-COOH in the first five days was about 1.3 to 1.4 days. With frequent use, the researchers then found half-lives of up to 10.3 days.

What is the difference between THC, 11-OH-THC and THC-COOH?

THC is the active substance, 11-OH-THC an active intermediate and THC-COOH the inactive end product of the breakdown. For the question of whether someone is under the influence right now, THC in the blood is what matters most. For the question of whether cannabis was used at all, detecting THC-COOH is enough.

After swallowing, for example with oils or capsules, the liver produces more 11-OH-THC than after inhaling.

THC in the blood: what do the values mean?

In blood, a distinction is made between active THC and the breakdown product THC-COOH. It is measured in blood serum and given in nanograms per millilitre (ng/ml). Since the change in the law in 2024, the limit for driving in Germany has been 3.5 ng/ml THC in blood serum (Section 24a(1a) of the Road Traffic Act, StVG). Before that, case law treated 1 ng/ml as reliable proof.

According to the German S3 guideline, after occasional use THC in serum is usually below 1 ng/ml after about 8 hours. With daily or several-times-daily use, values above 1 ng/ml can persist for longer than 24 hours. In a study with 30 men who used cannabis daily and stayed abstinent under supervision, THC was still measurable in the blood of some participants after 30 days.

THC-COOH in blood was used in driving licence practice as a hint about the pattern of use. In 2022, for example, the Bavarian Higher Administrative Court assumed regular use from 150 ng/ml THC-COOH in serum. Since 2024, however, the Driving Licence Regulation (Fahrerlaubnis-Verordnung, FeV) focuses on misuse and dependence for cannabis (Annex 4, No. 9.2 FeV). How a measured value is assessed in an individual case today is decided by the authority. More in the guide Driving with a cannabis prescription.

Does the limit also apply to patients?

There is an exception for patients, but no blank cheque. Under Section 24a StVG, the rules on fines do not apply if the THC comes from the intended use of a medicine prescribed for a specific medical condition. According to the S3 guideline, this applies to finished medicines as well as to medical cannabis flowers.

The exception does not apply in criminal law, and it does not cover combined use with alcohol if the THC was not prescribed. Anyone who feels impaired must not drive, whatever the value in the blood. What doctors say about driving during therapy and what the driving licence authority can check is explained in the guide Driving with a cannabis prescription.

How long is THC detectable in urine?

Urine tests look for THC-COOH and show use over the past days to weeks. The most precise study on this comes from Norway: prison inmates gave up to five urine samples a day over several weeks. In people with occasional use, the last positive sample was found after 4 days (lab confirmation, cut-off 15 ng/ml), 5 days (cut-off 10.3 ng/ml) or 12 days (rapid test, cut-off 20 ng/ml), depending on the method. With frequent use, the values were 17, 22 and 27 days.

With heavy continuous use it can take even longer. In an older study with 86 people with chronic use, the urine test stayed positive under strict supervision for up to 46 consecutive days; on average, excretion took 27 days. A typical finding was an alternation of negative and positive samples without any new use.

The concentration of the urine also plays a role. Drinking a lot dilutes the sample. Laboratories therefore often adjust for the creatinine content or treat heavily diluted urine as conspicuous.

Drug tests and rapid THC tests: how reliable are they?

A rapid THC test is a screening test, not proof. It uses antibodies that react to THC-COOH and related substances and shows whether a cut-off value has been exceeded. In the Norwegian study, only 78 percent of positive rapid tests were confirmed in the laboratory. That is why a positive rapid test with consequences for a driving licence, work or custody is checked in the laboratory with a second, chemically precise method.

Passive smoking only leads to positive urine tests under extreme conditions. In one experiment, non-users sat with users in a closed, unventilated chamber. At cut-offs of 75 and 100 ng/ml no test was positive, at 50 ng/ml exactly one, at 20 ng/ml several. With ventilation, exposure fell markedly, and positive results only occurred in the hours directly afterwards.

1. Positive rapid test: the result is preliminary. With the police or an employer, a blood or urine sample for the laboratory usually follows.
2. Laboratory analysis: a chromatographic method confirms THC-COOH or THC and measures the amount.
3. Assessment: whether a result has consequences, and which, depends on the context, for example road traffic, employment law or the driving licence.
4. If you are treated with medical cannabis: show your prescription or medication plan. What a patient ID can and cannot do is explained in the article Cannabis patient ID in Germany.

Saliva test: how long is THC detectable in saliva?

Saliva tests mainly show use within the last hours up to about a day. In traffic checks they serve as a preliminary test; a blood sample then establishes the value in serum. After smoking, THC goes directly into the oral mucosa, so values are very high at first and then fall quickly.

In a controlled study, people with occasional and with frequent use each smoked a joint with 6.8 percent THC. All saliva samples were positive up to 13.5 hours afterwards. With very low cut-offs of 1 to 2 ng/ml, THC remained detectable for 30 hours and longer. With frequent use, THC-COOH was also found for more than 30 hours.

How long is THC detectable in hair?

Hair stores a record covering months. Scalp hair grows roughly one centimetre per month, so for proof of abstinence a strand of hair is cut into segments. Each segment stands for about one month.

After stopping, new hair segments do not become negative straight away, because resting hairs still contain substances. In a study with 15 people after long-term cannabis use, the hair samples only became negative about six to seven months after use ended.

Hair analysis has an important weakness: THC and even THC-COOH can get into the hair without one's own use, for example via the hands, sweat and skin oil of people who use cannabis, or via smoke. Forensic scientists in Freiburg showed in 2015 that THC is hardly incorporated into hair after swallowing, but that THC-COOH also appeared in hair segments that had grown before the intake. A hair result therefore always has to be assessed in context.

Proof of abstinence via hair or urine mainly plays a role for the driving licence. After dependence, Annex 4 of the Driving Licence Regulation generally requires one year of proven abstinence.

Does THC show up in a blood count?

No. A small or full blood count counts and assesses blood cells such as red and white blood cells and platelets. Drugs or medicines are not measured.

Finding THC in blood requires a separate toxicological analysis that has to be ordered specifically. In a routine blood test at your GP's practice, THC is therefore not determined in passing.

Detection is not the same as effect

The effect of THC ends much earlier than its detectability. After inhaling, the effect begins within seconds to minutes, peaks after 15 to 30 minutes and wears off after 2 to 3 hours. After swallowing, it begins after 30 to 90 minutes, is strongest after 2 to 3 hours and lasts 4 to 12 hours depending on the dose.

For driving, the S3 guideline states that effects relevant to traffic generally wear off within 4 to 5 hours after inhaling. Specialist societies advise waiting 6 to 12 hours before driving after a single use. With daily use this is often not enough, because THC remains elevated in the blood for longer.

Conversely, a urine result after two weeks does not prove impairment at the time of the sample. That is exactly why test, measured substance and timing have to be kept clearly apart.

What affects how long THC stays detectable?

Frequency of use has the strongest effect, followed by the test method and cut-off value. General figures found online rarely take this into account. The following factors shift the time window.

• Frequency and duration: daily use over months builds up a large store in the tissue that keeps releasing THC for a long time.
• Amount and THC content: more THC per use means higher starting values.
• Route of intake: swallowing produces more breakdown products than inhaling.
• Sample: blood shows hours to days, urine days to weeks, hair months.
• Cut-off value: a test with 20 ng/ml finds THC-COOH for longer than one with 50 ng/ml.
• Dilution of the urine: a lot of fluid lowers the concentration, but is noticed in the laboratory via the creatinine value.
• Individual factors: body fat, metabolism and exercise influence release from the tissue.

Can you speed up THC elimination?

Not reliably. Release from fatty tissue and breakdown in the liver cannot be controlled in a targeted way. Drinking a lot dilutes urine only briefly at most, which the laboratory notices. So-called detox products sold online have not been tested.

The only reliable way to a negative result is time without use. How long depends on the factors above. If you are tested while being treated with medical cannabis, disclose your prescription rather than trying to get around a test.

What patients should know

If you take medical cannabis or a finished medicine containing THC, you will test positive. That is to be expected and not misconduct. Helpful are a copy of the prescription, an up-to-date medication plan and the medicine in its original packaging.

Separate rules apply at work, for example on consent, the works council and risk assessment. For driving, the exception for prescribed medicines only protects you with intended use and without impairment.

If you want to end a therapy, for example to provide proof of abstinence, discuss this with your doctor. Stopping abruptly can trigger withdrawal symptoms.

Frequently asked questions