If the question is "did this person use cannabis in the last day or two," blood is the sample that answers it. If the question is "has this person used cannabis at any point in the past three months," hair is the sample that answers it. Neither matrix is better in the abstract, because they answer different questions. The criteria that matter are the detection window each matrix covers, the analyte the lab actually measures, the cutoff concentration used for a positive result, and how much the result can tell you about timing. One shared limit applies to both: a positive test shows exposure, not impairment, and no lab result proves a person was high at a given moment.
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Blood: the short window for recent use
Delta-9-THC appears in whole blood within minutes of inhaling, peaks fast, and drops quickly. Because the parent drug clears so fast, most labs confirm with 11-nor-9-carboxy-THC (THC-COOH), the metabolite that lingers longer in circulation. For someone who uses once, THC-COOH is usually detectable in blood for about 1 to 2 days. For daily, heavy users, that window stretches to roughly 3 to 7 days, and a small number of very heavy chronic users may show detectable levels slightly beyond that. Edibles and concentrates tend to extend blood detection compared with a single smoked dose, since absorption is slower and the dose is larger.
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Pros
- Best available lab marker for very recent use
- Narrow window means a positive result points to the last few days, not the last few months
- Well studied, with established metabolites and standard cutoffs
Cons
- Invasive collection requiring a trained phlebotomist
- Short window makes it easy to miss use that happened last week
- Timing of the draw matters, since levels fall hour by hour after inhalation
Hair: the long window for cumulative exposure
Head hair grows at roughly 1 centimeter per month, or about half an inch. A standard sample of 1.5 inches (about 3.9 centimeters) taken from the scalp therefore covers about 90 days of growth. As cannabis metabolites circulate, traces deposit in the growing hair shaft, where they stay locked in place. Segmenting the strand lets a lab narrow the window: each half inch of hair corresponds to approximately one month, so a lab can say "this month" or "two months ago" rather than only "sometime in the last quarter." Hair cutoffs are measured in picograms per milligram, with 1 pg/mg of THC-COOH a common confirmation threshold and some labs screening lower.
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Pros
- Longest standard detection window of the common matrices, about 90 days
- Segmentation gives a rough monthly timeline of use
- Collection is non-invasive and hard to substitute on short notice
Cons
- Cannot pinpoint a day or a time of use
- External contamination from smoke, hands, or shared surfaces can raise results, so labs may wash samples and run extra steps
- Bleaching, dyeing, relaxing, and repeated heat styling can lower or alter what is detected
- Body hair grows at a different rate, so a body hair sample uses different timing assumptions
Blood versus hair, side by side
- Detection window: blood about 1 to 7 days; hair about 90 days from a 1.5 inch sample
- Analyte: blood typically THC-COOH plus parent THC; hair typically THC-COOH and related metabolites
- Timing detail: blood shows recent use with hour-level relevance; hair shows a monthly band at best
- Collection: blood needs a needle draw; hair needs a scalp sample cut close to the skin
- Main weakness: blood misses older use; hair is vulnerable to contamination and cosmetic damage
What changes the window in practice
- Frequency and dose. Chronic daily use builds up more metabolite than occasional use, which stretches both windows.
- Route. Edibles and concentrates generally produce longer blood detection than a single shared joint.
- Cutoff concentration. A lab using a very low threshold reports more positives than one using a higher confirmatory cutoff on the same sample.
- Body composition and metabolism. THC is fat soluble, so individual clearance varies widely.
- Hair treatment. Chemical processing damages the shaft and can reduce measurable metabolite.
Which test fits which situation
Choose blood when the decision hinges on the last 24 to 72 hours: a workplace incident, a suspected recent relapse, or research that needs a current measure. Choose hair when the question is about a pattern of use across weeks or months, such as a pre-employment screen or a monitoring program that wants a longer lookback. If you need a rough middle ground, urine usually covers about 3 days for occasional use and up to 30 days for heavy use, and oral fluid covers roughly 1 to 3 days. Each matrix has a job, and the right one depends on the time frame you care about.
Common questions
Can a blood test tell when someone used cannabis?
It can narrow the window to the last day or few days, but it cannot name an hour. Levels rise and fall quickly, so the draw time drives the result.
Does one hair test prove daily use?
No. It shows exposure during the growth period the strand covers. A segmented result can suggest repeated exposure across months, but it does not count uses or measure impairment.
Can hair results be challenged?
Yes. Contamination and cosmetic treatment are recognized limitations, which is why labs may wash the sample, test multiple segments, or confirm with a second method.