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How Long Can THC Stay in Your Hair?

When it comes to drug testing, hair analysis is often considered the most long-term method. While urine or blood tests detect recent cannabis use, hair testing can reveal usage over weeks or even months. But how long exactly does THC, the psychoactive component of cannabis, stay in your hair?

This article is for people facing workplace drug screening, those curious about how cannabis is traced in the body, or anyone looking for accurate information on hair testing. We explain how THC is detected in hair, how far back the results can go, and what UK standards say about cannabis testing.

What Is THC and How Does It Enter the Hair?

THC (tetrahydrocannabinol) is the chemical in cannabis responsible for the high. When cannabis is consumed, THC enters the bloodstream and is broken down into various metabolites, including THC-COOH. These by-products circulate throughout the body and can be deposited in growing hair follicles through small blood vessels in the scalp.

As your hair grows, these metabolites become trapped in the hair shaft. This means that a sample of hair can hold a record of THC exposure for weeks or even months, depending on how much hair is tested.

How Long Can THC Be Detected in Hair?

Hair drug testing typically looks at a 90-day window of drug use. This is based on the average growth rate of scalp hair, which is about 1 centimetre per month. Most tests use a 3 to 4 centimetre section of hair cut from close to the scalp, which gives a snapshot of cannabis use over the past three months.

However, in theory, THC metabolites can remain in hair for as long as the hair remains uncut. A person who has used cannabis in the past six months could still test positive if a long enough section of hair is analysed. This makes hair testing the most extended detection method for THC currently available.

What Affects How Long THC Stays in Hair?

Several factors influence how long THC can be detected in hair, including:

Frequency of use: Occasional users may have undetectable levels, while frequent users are more likely to produce a positive result
Hair length: The longer the hair, the further back a lab can trace usage if more is analysed
Hair type and colour: Darker hair may retain more metabolites due to higher melanin content
Metabolism and body chemistry: Individuals process and deposit THC at different rates
Environmental exposure: Although rare, second-hand smoke can contribute to external contamination, but most tests include a washing step to remove surface-level residue

What Does a Positive Hair Test Mean?

A positive result on a hair drug test indicates that THC or its metabolites were incorporated into the hair as it grew, which points to actual ingestion rather than passive exposure. It does not indicate intoxication at the time of testing, nor does it confirm when exactly the cannabis was consumed.

Hair tests are often used in situations where a long-term pattern of use is under scrutiny, such as:

Pre-employment screening
Random workplace drug testing
Family court or child custody proceedings
Rehabilitation or clinical assessments

Can You Remove THC from Hair?

There are many products marketed online that claim to remove drug residues from hair, but there is no proven method to reliably eliminate THC metabolites from hair once they are embedded in the shaft. Most drug testing labs use sophisticated washing, extraction and analysis processes that go beyond the surface.

The only guaranteed way to avoid THC showing up in a hair test is to stop using cannabis and allow time to pass. Since hair grows slowly, it can take several months for a new, THC-free segment to emerge.

Is Hair Testing Used in the UK?

Yes. While urine and saliva tests are more common for routine workplace testing in the UK, hair testing is used in more formal or sensitive situations where long-term drug use needs to be assessed. This may include occupational health evaluations, court orders or private testing.

Because THC remains a Class B controlled drug under the Misuse of Drugs Act 1971, its presence in hair may have legal or professional implications, depending on the context.

What If You Use Medical Cannabis?

In the UK, cannabis-based medicines containing THC may be prescribed by a specialist doctor. If you are undergoing a hair drug test and are using a legal prescription product, it is important to inform the testing organisation and provide evidence of your prescription. However, many workplace drug policies do not currently account for medical cannabis, and you may still face restrictions depending on the employer’s stance.

Conclusion

THC can stay in your hair for up to 90 days, and potentially longer if a longer section of hair is analysed. While hair testing cannot show when cannabis was used or whether someone was intoxicated at a particular time, it is a reliable way to detect long-term or repeated use.

If you are subject to hair drug testing and have used cannabis recently, it is likely that THC will be detected. The only way to avoid a positive result is to abstain from use and allow time for THC-containing hair to grow out and be trimmed away.