--- title: "Does marijuana show up on a blood test — and how long does THC stay detectable?" type: prep url: "https://www.test-well.com/prep/cannabis-thc" factor: Cannabis (THC and CBD) aliases: - does weed show up in blood work - how long does thc stay in urine - cbd drug test - marijuana before blood test reviewed_at: 2026-10-07 review_status: reviewed date_published: 2026-10-07 --- # Does marijuana show up on a blood test — and how long does THC stay detectable? > Routine blood work does not test for marijuana: a CMP, CBC, lipid panel, thyroid or hormone test will not detect or report THC, and cannabis use does not meaningfully change those results — though an edible the night before a fasting draw breaks the fast. Drug screens are different and are ordered on purpose. A standard urine screen looks for THC's metabolite and detects occasional use for about 1–3 days, regular use for 1–2 weeks, and daily heavy use for 30 days or more; hair tests reach back about 90 days; blood and saliva detect recent use (hours to a day or two). Legal, medical or prescribed cannabis still produces a positive result, and many CBD products contain enough THC to trigger one. Second-hand smoke rarely does at standard cutoffs. ## Which results does it affect? | Marker | Direction | How long the effect lasts | What to do | Source | | --- | --- | --- | --- | --- | | Routine chemistry, CBC, lipids, thyroid, hormones | interferes | Not tested for and not meaningfully changed | Nothing to do; an edible before a fasting draw breaks the fast like any food | [MedlinePlus — Drug testing (what urine screens detect and for how long)](https://medlineplus.gov/lab-tests/drug-testing/) | | Urine drug screen (THC metabolite) | raises | Occasional use ~1–3 days; regular ~1–2 weeks; daily heavy use 30+ days | Detection depends on frequency and body fat, not the last dose alone | [Mayo Clinic Laboratories — Marijuana (THC) detection windows](https://www.mayocliniclabs.com/test-info/drug-book/marijuana.html) | | Hair drug test | raises | About 90 days | Reflects repeated use, not a single exposure | [MedlinePlus — Drug testing (what urine screens detect and for how long)](https://medlineplus.gov/lab-tests/drug-testing/) | | Blood or saliva THC | raises | Hours to a day or two | Used for recent use / impairment contexts | [Mayo Clinic Laboratories — Marijuana (THC) detection windows](https://www.mayocliniclabs.com/test-info/drug-book/marijuana.html) | | CBD products | can raise | Full-spectrum products contain THC; detection as above | Assume a THC screen can be positive unless the product is verified THC-free | [MedlinePlus — Drug testing (what urine screens detect and for how long)](https://medlineplus.gov/lab-tests/drug-testing/) | ## Blood work vs a drug screen: two different tests Nothing in a standard metabolic, blood-count, cholesterol or hormone panel measures cannabis, and the laboratory does not add a drug screen to a blood test unless one is ordered. So the common worry — 'will my doctor see weed in my blood work?' — is answered by what was ordered, not by what you used. Drug screens are separate tests, usually on urine, specifically ordered for employment, legal, treatment or safety reasons, and they target THC's metabolite along with other drug classes. The TestWell drug screens are listed as such, and a routine panel never includes one. ## How long THC stays detectable THC's metabolite is stored in fat and released slowly, so the window depends far more on how often you use than on when you last did: a single or occasional use is typically detectable in urine for one to three days, regular use for one to two weeks, and daily heavy use for a month or longer. Hair testing reaches back roughly 90 days; blood and saliva reflect the last several hours to a day or two. Drinking water does not reliably clear a screen and can flag the sample as dilute. Medical-card and legal-state cannabis test the same as any other, and full-spectrum CBD products often contain enough THC to screen positive — if that matters, use a product tested THC-free. ## Common questions ### Does marijuana show up on regular blood work? No. CMP, CBC, lipid, thyroid and hormone tests do not test for THC and are not meaningfully changed by cannabis. Only a drug screen, which is ordered separately, detects it. ### How long does THC stay in your urine? About 1–3 days after occasional use, 1–2 weeks with regular use, and 30 days or more with daily heavy use — the window depends on frequency and body fat more than the last dose. Hair tests reach back about 90 days. ### Can CBD make you fail a drug test? Yes, if the product contains THC — many full-spectrum CBD products do. Use a product verified THC-free if a screen matters to you. ## Related - [Do you need to fast before a blood test — and which tests does eating affect?](https://www.test-well.com/prep/fasting) - [Does biotin affect blood test results?](https://www.test-well.com/prep/biotin) - [How does alcohol affect blood test results?](https://www.test-well.com/prep/alcohol) - [Should you exercise before a blood test?](https://www.test-well.com/prep/exercise) - [All preparation guides](https://www.test-well.com/prep) - [Lab visit guide](https://www.test-well.com/at-the-lab) ## Sources - [MedlinePlus — Drug testing (what urine screens detect and for how long)](https://medlineplus.gov/lab-tests/drug-testing/) - [Mayo Clinic Laboratories — Marijuana (THC) detection windows](https://www.mayocliniclabs.com/test-info/drug-book/marijuana.html) --- Medically reviewed by Youmna DiStefano, MD (Medical Reviewer, TestWell); last reviewed 2026-10-07. Review policy: https://www.test-well.com/reviewers/youmna-distefano-md Canonical page: https://www.test-well.com/prep/cannabis-thc Machine-readable catalog: https://www.test-well.com/api/catalog.json · MCP server: https://www.test-well.com/mcp · Overview for agents: https://www.test-well.com/llms.txt Prices are cash-pay, in USD, and include physician authorization; every order adds one flat $6 lab processing fee. Educational content is not a diagnosis — discuss results with a clinician.