The short answer: a standard hair follicle test can detect cannabis use going back about 90 days, roughly three months, when the sample comes from the head. If a lab collects body hair instead, the window stretches much further, in some cases up to 12 months. That estimate assumes a typical head hair sample of around 1.5 inches taken from the crown, a lab cutoff that matches the employer or agency policy, and a person whose hair grows at an average rate. Every one of those variables can move the number in either direction, so 90 days is a benchmark rather than a guarantee.
can cannabis be detected in hair after one use
Why 90 Days Is the Standard Benchmark
Head hair grows at roughly half an inch per month, or about 1.25 centimeters. When a lab snips a 1.5-inch segment closest to the scalp, it is effectively reading a timeline of the last three months. Drugs and their metabolites enter the hair shaft through the blood supply at the follicle, and once they are locked into the keratin structure, they stay there as the hair grows out.
How Long Do Cannabis Metabolites Stay in Hair?
That growth math is why hair testing is popular with employers and courts. A urine test covers days to a few weeks. A hair test covers a full quarter. For cannabis specifically, the lab is usually looking for THC and its metabolites rather than the parent plant material.
What Changes the Detection Window
- Hair length. A longer sample can represent a longer timeline, though most labs only test the most recent 1.5 inches. Some programs test a longer segment when a wider history is requested.
- Hair source. Body hair grows slower than scalp hair, so a body hair sample can reflect a year or more of use.
- Growth rate. Individual growth rates vary with age, genetics, health, and some medications.
- Use pattern. Heavy, daily, or high-potency use tends to deposit more detectable metabolite than a single occasional session.
- Lab cutoff. Two labs can test the same sample and reach different conclusions if their screening and confirmation thresholds differ.
- Metabolism and body composition. These matter more for urine and blood than for hair, but they still shape how much metabolite reaches the follicle.
What a Hair Test Actually Looks For
Most cannabis panels screen for THC and its breakdown products, and some also include cannabinol and cannabidiol. A screening immunoassay gives a preliminary result. Any non-negative screen is normally confirmed with a mass spectrometry method such as GC-MS or LC-MS/MS, which can separate THC from closely related cannabinoids and reduce false positives.
How Long Does Cannabis Stay in Hair?
The Recent-Use Blind Spot
Hair has to grow out of the follicle before it can be collected, so use within roughly the last week may not appear in the sample. That creates a strange gap where a hair test can miss what happened yesterday but catch what happened two months ago. It also means the first inch of hair near the scalp is the freshest material a lab can read.
Does CBD Show Up on a Hair Test?
CBD itself is not usually the target of a workplace cannabis screen. The risk comes from THC. Full-spectrum CBD products can carry small amounts of THC, and repeated daily use of a full-spectrum oil over months is the scenario most likely to build up detectable metabolite. Broad-spectrum and isolate products are formulated to remove THC, but the label is only as good as the testing behind it.
Independent third-party lab reports are the practical check here. Look for a batch-specific certificate of analysis that lists cannabinoid content, including total THC, and states the testing laboratory. A certificate that shows only "CBD content" without a THC line tells you very little. Regulators have repeatedly found CBD products whose actual THC content did not match the label, so a certificate of analysis from an accredited lab is the most reliable signal available to a buyer.
What Does Not Work
- Detox shampoos. Marketing claims outrun the evidence, and labs now screen for common adulterants.
- Bleaching or dyeing. This damages hair and can trigger a collection of body hair instead, which widens the window rather than closing it.
- Short notice abstinence. Once metabolite is inside the shaft, stopping use does not remove it. Only new growth is clean.
Practical Takeaways
- Assume a 90-day window for head hair and up to a year for body hair.
- Ask which lab, which specimen type, and which cutoff apply if you are in a testing program.
- If you use CBD and face testing, choose isolate or broad-spectrum products and read the batch certificate of analysis before you buy.
- Keep records of what you use. Knowing the THC content of your product is the only way to judge your own exposure.
None of this is medical or legal advice. Testing policies vary by employer, state, and agency, and the only way to know the rules that apply to you is to read the policy you are subject to or ask the program administrator directly.