How Long Does Cannabis Stay in Your Hair?

Cannabis detection in hair is a fascinating fusion of biology, chemistry, and forensics that is becoming increasingly advanced with hair follicle drug testing.

The biology of drug testing with hair

Cannabis, when consumed, eventually circulates THC metabolites through blood vessels to the hair follicle. As hair grows, those metabolites are embedded in the structure of the hair shaft forever.

Dr. Sarah Mitchell, a forensic toxicologist, says it this way: “Hair records biological events and captures a timeline of substance use, where the timeline is virtually irreversible once hair has grown out.”

What you need to know about hair detection timelines

Detection Window	Hair Length	Reliability
Up to 90 days	1.5 inches	Very High
Up to 6 Months	3 inches	High
Beyond 6 Months	Longer	Moderate

The standard procedure relies heavily on collecting the most recent 1.5 inches of hair nearest the scalp which is equal to about 90 days’ growth at an average rate of 0.5 inches per month.

Factors Affecting Detection

Individual Biology

• Rate of hair growth: Hair typically grows at a rate of 0.3 – 0.7 inches per month.
• Hair texture: Coarse hair will hold metabolite deposits better than finer hair.
• Natural hair color: Darker hair will retain metabolite concentrations longer because of melanin.

Usage

Your usage patterns can significantly impact the likelihood of detection.

• Frequency: Individuals who consume products daily will show detectable levels for the entire 90-day cycle.
• Amount: Regular heavy users will produce detectable concentrations of metabolites compared to a light occasional user, who may have no detectable concentrations of metabolites in their hair.
• Potency: Heavy users who consume high THC products will leave more obvious fingerprints than someone who consumes a lower THC product for the same duration and quantity.

Real example

Say for example, Jake, who was an intermittent consumer who consumed edibles two times in January. When he was tested in March (60 days from his last consumption), he had detectable THC metabolite concentrations in his hair sample even though he had consumed infrequently. To put it another way, while it is possible for a recent user to consume and test negative within a few weeks with urine testing, it would still be possible to have THC deposited in hair follicle testing until 90 days after the last consumption.

Testing Process and Accuracy

Today’s hair follicle testing typically uses technologically advanced methods of testing (for example, GC-MS (Gas Chromatography Mass Spectrometry) to test for specific metabolite deposits. The process includes:

• Hair sample collection: 100 – 120 hair samples from multiple collection sites on the scalp.
• Hair decontamination: External decontamination of the hair sample to eliminate external environmental contaminants.
• Hair digestion: Breaking down the inner structure of the hair to release the metabolites of THC.
• Analyzing the sample: Identifying the levels of THC-COOH in the hair.

As a threshold level, the cut-off is typically 1.0 pg/mg so that passive exposure would not result in a false positive in the test.

Limitations

Hair testing can detect between three months to 90 days of previous use within the standard protocol for testing, depending on the length of the hair and how long the last use was prior to testing. Also, it is possible for a person who has shaved their body hair or has no body hair to complicate collection altogether.

Cannabis metabolites can be identified in hair testing for as long as the standard 90 days for testing is concerned. However, potentially, if the hair sample is an extended length, the metabolites could last more than 90 days. The advantage of hair over other sample testing methods is that, unlike urine, saliva, or blood tests, the analysis of cannabis in hair provides almost historical information of cannabis use.

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