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How long does THC stay in your blood?

By The reference desk · Fact-checked against the cited sources

Published · Sources checked Oct 4, 2026

Short answer

Blood tests measure THC itself. After one cannabis cigarette, occasional smokers fell below 5 ng/mL within about 2 hours. In some daily users, studies found THC in blood for 7 days or more, and in a few for up to 30 days.

Reported detection windows for THC, by test and by how often someone uses. Ranges come from published studies, mostly of smoked cannabis. Sources checked Oct 4, 2026.
TestOnce or now and thenSeveral times a weekDailyHeavy, long-term
BloodHours (above 5 ng/mL: about 1 to 2 hours)5 Not well studied 7 days or more in some people6 Up to 30 days in some people7

What the test measures

A blood test measures THC itself, in nanograms per millilitre (ng/mL, the same as µg/L). Federal workplace programs test urine and oral fluid, not blood.[1][2]

Typical windows

How often Reported window
Once, or now and then Hours. Above 5 ng/mL, the last positive came at a median of 1 hour (range 0 to 2.1 hours)[5]
Several times a week Not well studied
Daily 7 days or more in some people[6]
Heavy daily use for months Up to 30 days in some people[7]

The studies behind the ranges

  • One cigarette, occasional and frequent smokers. In a NIH study, 11 occasional smokers stayed above 5 ng/mL for a median of 1.0 hour (range 0 to 2.1 hours) after one cannabis cigarette. Frequent smokers stayed above it for a median of 3.5 hours, and one for more than 30 hours.[5]
  • Dependent users in treatment. Among 21 people entering residential treatment, THC was detectable in the blood of some for at least 7 days.[6]
  • Chronic daily smokers held abstinent. Of 30 chronic daily smokers on a NIH research unit, 27 had THC in blood on admission. Levels fell slowly: only 1 of 11 was negative at 26 days, and 2 of the 5 still in the study had THC at 0.3 ng/mL after 30 days.[7]

Why the numbers differ so much

THC is stored in body fat and released back into the blood slowly. The more often and the longer someone has used, the larger that store, so the same low level can mean last use a few hours ago in one person and weeks ago in another.[7] The authors of the 30-day study raised this as a problem for driving laws that set a fixed blood THC limit.[7]

Before you rely on a number

Cutoffs differ between labs and between purposes. A range from a study is not a prediction for one person; only a lab test can tell you your result.

Questions

Why is the blood window so short after one use?

THC moves out of the blood into body fat and other tissue quickly, so blood levels after a single use fall within hours.

Why can daily users stay positive for weeks?

In chronic daily smokers held abstinent, THC stored in the body kept returning to the blood at low levels. A NIH study measured it for up to 30 days in some participants.

Does a blood level show impairment?

No. This site does not interpret impairment. In daily users, low blood levels persisted for weeks after last use, so a level alone does not show when someone used.

Sources

Sources checked Oct 4, 2026. Numbers in the text link here.

  1. Substance Abuse and Mental Health Services Administration. Mandatory Guidelines for Federal Workplace Drug Testing Programs. Federal Register 82 FR 7920, January 23, 2017. www.federalregister.gov/d/2017-00979Checked Oct 4, 2026 against the full text: the cutoff table.
  2. Substance Abuse and Mental Health Services Administration. Mandatory Guidelines for Federal Workplace Drug Testing Programs: Oral Fluid. Federal Register 84 FR 57554, October 25, 2019. www.federalregister.gov/d/2019-22684Checked Oct 4, 2026 against the full text: the cutoff table and the THC discussion.
  3. Desrosiers NA, Himes SK, Scheidweiler KB, Concheiro-Guisan M, Gorelick DA, Huestis MA. Phase I and II cannabinoid disposition in blood and plasma of occasional and frequent smokers following controlled smoked cannabis. Clin Chem. 2014;60(4):631-643. doi.org/10.1373/clinchem.2013.216507Checked Oct 4, 2026 against the abstract.
  4. Odell MS, Frei MY, Gerostamoulos D, Chu M, Lubman DI. Residual cannabis levels in blood, urine and oral fluid following heavy cannabis use. Forensic Sci Int. 2015;249:173-180. doi.org/10.1016/j.forsciint.2015.01.026Checked Oct 4, 2026 against the abstract.
  5. Bergamaschi MM, Karschner EL, Goodwin RS, Scheidweiler KB, Hirvonen J, Queiroz RH, Huestis MA. Impact of prolonged cannabinoid excretion in chronic daily cannabis smokers' blood on per se drugged driving laws. Clin Chem. 2013;59(3):519-526. doi.org/10.1373/clinchem.2012.195503Checked Oct 4, 2026 against the abstract.