Blood Tests for THC: What They Show
Blood tests measure THC itself rather than only a leftover metabolite, so they sit closer to recent use. But in people who use daily, THC can stay measurable in blood for weeks after they stop. Usually a short window: Plasma detection is often 1–2 days, and blood generally reflects use within the past day.
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Key Takeaways
- Usually a short window. Plasma detection is often 1–2 days, and blood generally reflects use within the past day.
- Daily users are the exception. In 30 chronic daily smokers held abstinent, 2 of 5 still had measurable THC after 30 days.
- Not a federal workplace specimen. Federal workplace programs authorize only urine and oral fluid.
What blood measures
Blood can be tested for THC and its metabolites, including the active 11-hydroxy-THC and inactive THC-COOH. THC itself has a short clearance half-life (under 30 minutes, per Mayo Clinic Laboratories), so in occasional users blood levels fall quickly. StatPearls describes plasma detection as often 1–2 days and notes blood and saliva generally reflect recent use, within about 24 hours.
Chronic daily users can test positive for weeks
A NIDA intramural study followed 30 male chronic daily cannabis smokers who lived on a secure research unit for up to 33 days without using. On admission, 27 were THC-positive, with a median of 1.4 µg/L. THC fell gradually: only 1 of 11 participants was negative at 26 days, 2 of 5 remained THC-positive (0.3 µg/L) at 30 days, and 4 of 5 were still positive for THC-COOH after 30 days.
The authors concluded cannabinoids "can be detected in blood of chronic daily cannabis smokers during a month of sustained abstinence," which matters for per se drugged-driving laws that set a fixed blood THC limit.
Where blood tests come up
Blood is not part of federal workplace testing, which authorizes only urine and oral fluid. It comes up mostly in clinical settings and in impaired-driving cases, where some states use blood THC thresholds (the reason the Bergamaschi study was framed around per se driving laws). As with every cannabis test, a positive confirms exposure, not impairment.
If you are weighing when it is safe to drive after using, a blood test window is the wrong yardstick. See how long a high lasts and your state's rules in our laws hub.
Common questions
How long does THC stay in your blood?
For occasional use, usually 1 to 2 days, and blood generally reflects use within about the past day. In chronic daily users it can last much longer: in one study, 2 of 5 participants still had measurable THC after 30 days of abstinence.
Do employers use blood tests for cannabis?
Federal workplace programs do not. HHS guidelines authorize only urine and oral fluid specimens. Blood testing is more common in clinical and impaired-driving contexts.
Sources
- study StatPearls (NCBI Bookshelf), Clinical Drug Testing ↗ accessed 2026-10-03
- study StatPearls (NCBI Bookshelf), Cannabis Use Disorder ↗ accessed 2026-10-03
- lab Mayo Clinic Laboratories, Drug Book: Marijuana ↗ accessed 2026-10-03
- study Bergamaschi MM et al., Impact of Prolonged Cannabinoid Excretion in Chronic Daily Cannabis Smokers’ Blood on Per Se Drugged Driving Laws, Clinical Chemistry 2013;59(3):519-526 ↗ accessed 2026-10-03
- government SAMHSA, Drug-Free Workplace: Frequently Asked Questions ↗ accessed 2026-10-03