Adults only (18+)

Does THC Show Up in Bloodwork?

As cannabis use becomes more widely discussed and tested for, particularly in the context of driving, employment or medical screening, many people want to know whether THC, the psychoactive compound in cannabis, shows up in blood tests. The short answer is yes. THC can be detected in blood, but how long it remains detectable depends on several factors including how much was used, how often, and how the body processes it.

This article is for individuals curious about cannabis testing, those facing workplace or legal drug screening, and anyone interested in how THC behaves in the body. We explain what bloodwork can reveal, how long THC stays in your system, and how UK testing standards apply.

What Is THC and How Does the Body Process It?

THC (tetrahydrocannabinol) is the primary psychoactive ingredient in cannabis. Once consumed, whether through smoking, vaping, eating or oils, THC enters the bloodstream and travels to the brain, where it affects perception, coordination and mood.

After entering the bloodstream, THC is quickly distributed to fatty tissues and organs. The body begins to metabolise it into other compounds, such as 11-OH-THC and THC-COOH, which are then gradually eliminated through urine and faeces. However, traces of THC and its metabolites can remain in the body for several days or even weeks.

Can THC Be Detected in Blood?

Yes, THC can be detected in a blood test, but its presence depends on:

How recently it was used: THC is detectable in the blood for a much shorter time than in urine or hair. In most cases, it is present in the blood for 3 to 12 hours after use. In heavy or chronic users, detection windows may extend to 24 hours or longer.

How it was consumed: Smoking and vaping deliver THC rapidly to the bloodstream, resulting in a higher initial concentration. Edibles take longer to be absorbed but may produce longer-lasting levels.

Frequency of use: Occasional users may eliminate THC faster, while daily or long-term users often retain detectable levels for longer periods.

Body composition and metabolism: THC is fat-soluble, so individuals with higher body fat percentages may store THC longer than leaner individuals. A faster metabolism may also reduce detection time.

Why Would a Blood Test Be Used?

Blood tests for THC are less common than urine tests, but they are often used in specific legal or medical scenarios where recent use needs to be confirmed. These include:

Drug driving investigations: In the UK, police may carry out blood tests if someone fails a roadside drug screening. It is illegal to drive with more than 2 micrograms of THC per litre of blood, even if you are not impaired.

Medical monitoring: In rare cases, THC levels may be measured for therapeutic monitoring, especially if a person is using cannabis-based medicines legally under prescription.

Workplace testing: Most UK employers rely on urine testing, not blood tests, for drug screening. However, some safety-critical roles may involve bloodwork following an incident or for confirmatory testing.

How Long Does THC Stay in the Blood?

The detection window for THC in blood varies depending on the type of user:

Occasional users: THC is typically detectable in blood for up to 12 hours, possibly up to 24 hours depending on dosage.

Frequent users: For those who use cannabis daily or near-daily, THC may be detectable in blood for 24 to 72 hours, and in rare cases, slightly longer.

While active THC is usually cleared from the blood within a few days, its metabolites can remain in the body and be detected by other means, such as urine or hair tests, for much longer.

Does a Blood Test Detect Impairment?

Blood tests for THC do not measure impairment directly. They measure the amount of THC or its metabolites in the blood at the time of the test. Impairment varies between individuals and can depend on tolerance, dosage and context.

In UK drug driving law, there is a zero-tolerance approach to illegal drugs such as THC. Even if you are not visibly impaired, a positive test above the legal threshold can lead to arrest, fines, licence suspension or a criminal conviction.

What About Prescription Cannabis?

In the UK, cannabis-based medicinal products containing THC can be prescribed by specialist doctors in certain cases, such as for multiple sclerosis or epilepsy. However, if you are using such a product and driving, you may still test positive in a blood test. The law provides a medical defence, but you must show that the drug was prescribed and used as directed. You must also not be impaired while driving.

If you are using THC-based medicine, always consult your doctor before driving or undergoing any form of drug testing.

Can You Speed Up THC Elimination?

There are no guaranteed methods to quickly remove THC from your system. Hydration, exercise and time are the main factors that help the body clear THC naturally. Detox products marketed online are unlikely to be effective and are not recommended by medical professionals.

Conclusion

THC can be detected in blood shortly after use, particularly in the first 12 to 24 hours. While blood tests are less commonly used than urine or saliva tests, they are preferred in legal and medical contexts where recent use matters, such as drug driving cases.

If you are using cannabis, whether recreationally or under prescription, it is important to understand how THC behaves in the body and how it might affect drug test results. While THC does not linger in the blood for as long as in other tissues, it can still lead to legal or employment consequences if detected at the wrong time.