Adults only (18+)

How Long Does THC Stay in Your Blood For?

As cannabis use becomes a more widely discussed issue in the UK, questions around how long THC stays in the body have become increasingly common. THC, short for tetrahydrocannabinol, is the psychoactive component in cannabis responsible for producing the high. While saliva and urine tests are more commonly used for workplace screening, blood tests play a key role in medical settings and drug driving cases.

This article is for individuals facing a blood test, anyone curious about how THC moves through the body or those seeking accurate information on detection times. We explain how THC enters the bloodstream, how long it stays there and how blood tests are used to detect cannabis use.

How Does THC Enter the Bloodstream?

When cannabis is smoked or vaped, THC rapidly passes through the lungs and into the bloodstream. From there, it travels to the brain and other organs, producing its psychoactive effects within minutes. When cannabis is eaten, the process takes longer because THC must pass through the digestive system first, but it still eventually enters the bloodstream.

Once in the blood, THC levels rise quickly and then drop as the body metabolises and stores the compound in fat cells. This means the concentration of THC in the blood does not stay constant and can fall even while a person still feels high.

How Long Can THC Be Detected in Blood?

THC stays in the blood for a shorter period than in urine or hair. However, the exact detection time depends on several factors, including how often the person uses cannabis and how much was consumed. Typical detection times are as follows:

Occasional users may show THC in their blood for up to 12 to 24 hours after use
Regular users may have THC present in the blood for up to 48 hours
Heavy or chronic users may still have trace levels for up to 7 days or more

The active compound THC usually drops to very low levels in the blood within a few hours. However, its metabolites, particularly THC-COOH, may remain for longer periods and can be picked up in more sensitive blood tests.

When Are Blood Tests Used to Detect THC?

Blood testing is more invasive and expensive than saliva or urine testing. Because of this, it is typically reserved for situations where accurate and immediate information is needed. These include roadside drug driving investigations, hospital emergency admissions and legal or forensic analysis.

In drug driving cases, UK police may first use a saliva test to check for cannabis. If a driver tests positive, they are taken to a police station or hospital for a blood sample. This sample is analysed for THC concentration to determine whether the driver has exceeded the legal limit.

What Is the Legal THC Limit for Driving in the UK?

In the UK, the legal limit for THC in blood is 2 micrograms per litre. This is one of the lowest thresholds among controlled substances and is designed to be stricter than limits for alcohol. It is possible to exceed this level even if you do not feel impaired, especially if you have used cannabis in the previous 12 hours.

Driving with THC above this limit is a criminal offence. Penalties include a driving ban, a fine, points on your licence and a criminal record. There is no legal defence for recreational cannabis use behind the wheel. However, individuals with a valid medical prescription for cannabis-based medicine may present a medical defence if they are not impaired and are following their prescribed treatment.

Can a Blood Test Show How Recently Cannabis Was Used?

Unlike hair or urine tests, blood testing provides a more accurate indication of recent use. THC levels rise within minutes of use and fall steadily after a few hours. This makes blood testing useful for identifying whether a person was under the influence at a particular time.

However, there is no universal timeline that links THC blood concentration with level of impairment. Some individuals may test positive for several hours after they no longer feel high, while others may be impaired at lower levels depending on tolerance and sensitivity.

What Affects THC Levels in the Blood?

Several factors influence how long THC stays in the bloodstream. The method of use matters. Smoking and vaping produce quicker spikes in THC, while edibles lead to slower and longer-lasting absorption. Frequency of use also plays a role. THC builds up in the body with regular use, increasing the time it takes to fully clear.

Metabolism varies between individuals. A faster metabolism may break down THC more quickly. Body fat also contributes. THC is stored in fat cells and can be slowly released back into the blood over time. These differences mean blood testing can vary significantly in how long it shows a positive result.

Conclusion

THC can stay in your blood for up to 24 hours if you are an occasional user and for several days if you are a regular or heavy user. Blood tests are typically used in legal and clinical settings because they can detect recent cannabis use with greater accuracy than urine or saliva tests.

In the UK, driving with even small amounts of THC in your blood can result in serious legal consequences. If you use cannabis for medical or recreational reasons, it is important to understand how THC is processed and how long it may remain in your system.