How Long Does Weed Stay in Your Hair?

Cannabis use leaves a trace in the body that goes far beyond the initial high. Whether you’ve smoked it once or use it regularly, THC—the active chemical in cannabis—can remain detectable in various parts of your system for days, weeks, or even months. In particular, drug testing can reveal past cannabis use long after the effects have worn off.

One of the longest-lasting traces of cannabis can be found in your hair, where it can stay for up to 90 days. But that’s just one piece of the puzzle. In this article, we’ll explore how cannabis shows up in hair, blood, saliva and urine, what affects how long it stays in your body, and what it means if you're subject to drug testing—especially at work.

Cannabis in Hair: How Long Does It Stay?

When you use cannabis, THC enters your bloodstream and is carried throughout your body—including your hair follicles. As your hair grows, chemical markers of THC become embedded in the strands. Because human hair typically grows about 1 cm per month, testing a 3 cm sample taken from the scalp can detect cannabis use going back up to 90 days.

Hair tests don’t detect immediate or very recent use (within the past few days), but they are widely used to establish patterns of longer-term cannabis consumption, particularly in workplace drug testing, custody cases, and legal matters.

If you shave your head, can you beat the test? Not necessarily. Some tests can use body hair, which grows more slowly and can reflect an even longer history of use. Shaving may raise suspicion and could be interpreted as an attempt to evade testing.

Cannabis in the Bloodstream

THC enters the bloodstream within minutes of smoking or vaping. If ingested (via edibles), it takes longer—typically 30 minutes to 2 hours. Once in the blood, THC is quickly absorbed by fat tissues, but metabolites (the chemicals your body breaks THC down into) can remain in the blood for a considerable time.

• Occasional users: Up to 1–2 days

• Regular users: Up to 7 days or longer

Blood tests are most often used in roadside or accident-related drug testing, where recent impairment is the concern. Because THC levels fall quickly after use, blood tests are more useful in detecting recent or current intoxication, rather than long-term use.

Cannabis in Saliva

Saliva testing is often used for roadside drug tests and workplace checks because it’s quick, non-invasive, and cost-effective. THC can be detected in saliva almost immediately after use.

• Occasional users: Up to 24–48 hours

• Frequent users: Possibly up to 72 hours

Saliva testing is designed to pick up recent use, so it’s common for results to return negative if the last use was several days ago. However, for people who use cannabis daily, even a few days off may not be enough to pass.

Cannabis in Urine

Urine tests are the most widely used drug testing method in the workplace. THC itself doesn’t show up in urine, but its metabolite (THC-COOH) does.

Detection times vary:

• Single use: Up to 3 days

• Moderate use (a few times per week): Up to 10 days

• Chronic use (daily): Up to 30 days or more

• Heavy long-term use: Possibly up to 60+ days

Because THC metabolites are stored in body fat, urine tests are more sensitive for regular users and those with higher body fat percentages.

Comparison Table: Cannabis Detection by Test Type

What Affects How Long Cannabis Stays in Your System?

Frequency of Use

The more often you use cannabis, the more it builds up in your fat cells. Occasional users may clear it in a few days, while daily users may take weeks or months to eliminate all traces.

Body Fat and Metabolism

THC is lipophilic, meaning it binds to fat. Individuals with higher body fat percentages may retain THC longer. A faster metabolism can help eliminate it more quickly.

Potency of the Cannabis

Strains with higher THC content—especially those labelled as "Cali weed" or premium imports—can leave behind more metabolites, increasing detection windows.

Method of Use

Smoking or vaping introduces THC quickly and leads to faster elimination than edibles, which take longer to digest and may linger in the system longer.

Hydration and Lifestyle

Staying well-hydrated and physically active may help speed up elimination, though these are not guaranteed solutions. Exercise shortly before a test can also release stored THC into the bloodstream, potentially increasing the chance of a positive result.

Workplace Drug Tests: What Happens If You Fail?

If you fail a workplace drug test in the UK, the consequences depend on your employer’s policy. Some companies may:

• Terminate employment

• Suspend you pending investigation

• Refer you to occupational health or counselling

• Issue a formal warning

In safety-critical jobs (e.g. construction, transport, aviation), zero-tolerance policies are more common. Drug testing is usually part of a company’s duty of care to protect workers and the public from impaired decision-making or performance.

Employers test for drugs not just for legality, but for health and safety, insurance compliance, and maintaining a reliable workforce.

How Long Do the Effects of Cannabis Last?

While THC can remain in your system for weeks, the psychoactive effects typically wear off after:

• Smoking/vaping: 1 to 3 hours

• Edibles: 4 to 8 hours (can be longer depending on dosage)

Some lingering effects—such as drowsiness or reduced concentration—may last into the next day, particularly after heavy use.

What If You Just Shave Your Hair?

Shaving your head doesn’t guarantee a clean slate. Most hair tests require 3cm of hair, but if scalp hair is not available, labs may use body hair, which grows more slowly and reflects a longer detection period.

Shaving may also be seen as suspicious behaviour, and employers or legal authorities may request alternate testing.

Long-Term Effects of Cannabis Use

Long-term or heavy cannabis use can lead to several physical and mental health issues, including:

• Memory and concentration problems

• Increased risk of anxiety and depression

• Dependency and withdrawal symptoms

• Impaired lung function (if smoked)

• Reduced motivation and productivity

• Worsened outcomes for users with pre-existing mental health conditions

While some argue cannabis is safer than other substances, long-term daily use carries risks, especially for young adults whose brains are still developing.

Where This Leaves Us

Understanding how long cannabis stays in your system isn’t just about passing a drug test—it’s about knowing how your body processes substances, how usage habits influence those outcomes, and what that means in real-world terms.

Whether you're a casual user, a patient, or simply curious, it’s important to make informed decisions about cannabis—especially in a country like the UK, where it remains a controlled substance with serious legal implications.

If you're facing a test, considering use, or working in a sensitive role, stay informed, stay cautious, and always think ahead.