THC detection times at 50 ng/mL: what the cutoff means (and why timelines vary)
If you’re anxious about a marijuana urine test, you’ll see “50 ng/mL” everywhere. This page explains what that number means, how confirmation testing works, and what science says about why THC metabolite detection times can range from days to weeks.
What does “50 ng/mL” mean on a marijuana urine test?
50 ng/mL is a cutoff concentration used in many urine screening tests for marijuana. It’s a reporting threshold — not a measure of impairment, safety, or how “high” someone is.
Most urine tests look for THC metabolites (especially THC-COOH, sometimes labeled THCA or carboxy-THC), because THC itself clears from the blood relatively quickly while metabolites can persist longer.
Key point: A urine cannabinoid test mainly indicates prior exposure, not current impairment or an exact “last-used” timestamp.
Some laboratories use different marijuana cutoffs (commonly 20, 50, or 100 ng/mL), so “how long” can change depending on the testing program and lab method. If your test is for a regulated program (e.g., transportation), the cutoff rules may be standardized.
Related: THC drug testing for urine, blood, hair, and saliva
How long does THC stay detectable at 50 ng/mL?
There’s no single number that applies to everyone. Detection depends on dose, frequency, body composition, metabolism, hydration, the specific test method, and the cutoff used. Even “typical” ranges can’t guarantee what will happen for an individual test.
| Use pattern | What studies/guidance suggest at or near a 50 ng/mL screen | What to remember |
|---|---|---|
| Single or one-time use | Research reviews describe a relatively narrow window at a 50 ng/mL immunoassay cutoff — often around 1–2 days (average values around ~42 hours have been reported in controlled settings). | Potency, dose, and individual variability can extend this. Lower cutoffs (like 20 ng/mL) generally detect longer than 50 ng/mL. |
| Infrequent / occasional use | Guidance for federally influenced programs notes that infrequent use may produce positive initial results for roughly 1–5 days. | Timing is not predictable enough to “count days.” If stopping is difficult, support is available. |
| Regular / frequent use | THC-COOH can accumulate in the body with repeated use, and detection can extend into weeks for some people (especially with more sensitive cutoffs and confirmation testing). | Urine results do not reliably show impairment or the exact time of last use. |
| Chronic heavy daily use | Medical references commonly describe detection that can last multiple weeks and sometimes longer, depending on testing sensitivity and individual factors. | If you feel unable to stop, a structured treatment plan can help reduce relapse risk and improve outcomes. |
Practical takeaway: The only dependable way to avoid a positive result is to not use and allow time for natural clearance. Any claim that someone can “guarantee” a negative test on demand should be treated skeptically.
Why cannabis detection times vary so much
Even at the same 50 ng/mL cutoff, two people can have very different results. The biggest drivers include:
- Frequency & duration of use: repeated use increases body stores of THC metabolites.
- THC potency and dose: higher dose can extend detection.
- Body composition: THC is lipophilic (fat-soluble), which can contribute to longer persistence for some.
- Metabolism & overall health: people process substances differently.
- Testing method & cutoff: 20 vs 50 vs 100 ng/mL changes the detection window; confirmation cutoffs can differ from screening.
Also important: A positive urine test typically indicates prior use — it does not tell whether someone is currently impaired.
FAQ: 50 ng/mL THC detection times
Is 50 ng/mL the standard cutoff for marijuana urine tests?
It’s common, but not universal. Many programs use 50 ng/mL for screening, while some labs use 20 or 100 ng/mL depending on the policy and test panel.
Why do I see “15 ng/mL” mentioned for THC confirmation?
Many regulated programs use 50 ng/mL for the initial screen and a more specific confirmatory method with a 15 ng/mL cutoff for THCA/THC-COOH.
Can a urine THC test tell exactly when I last used?
No. Urine levels are affected by many factors (metabolism, accumulation, hydration), so a urine test generally can’t pinpoint the exact time of last use.
Can secondhand cannabis smoke cause a positive at 50 ng/mL?
In typical real-world situations, it’s considered unlikely at standard cutoffs, though extreme exposure scenarios are sometimes discussed in research and policy guidance.
What if I’m using cannabis and can’t stop (or keep relapsing)?
You’re not alone. Cravings, withdrawal, anxiety, and sleep disruption can make quitting hard. A structured plan (therapy, support, and in some cases medical care) can make stopping safer and more sustainable.
Need help stopping cannabis?
If marijuana use is starting to affect your work, relationships, mood, or ability to function — or if you’re stuck in a cycle of quitting and relapsing — treatment can help you regain control.
If you need immediate, confidential treatment referrals in the U.S., you can also contact SAMHSA’s National Helpline (24/7).
Sources & references
We prioritize primary/official references for cutoffs and testing interpretation, plus clinical/medical sources for general detection variability.
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U.S. Department of Transportation — 49 CFR Part 40, §40.85 cutoff concentrations (urine).
transportation.gov -
SAMHSA — Medical Review Officer Manual (workplace drug testing guidance; includes 50 ng/mL screening and 15 ng/mL confirmation cutoffs).
samhsa.gov (PDF) -
CDC — “Urine Testing for Detection of Marijuana: An Advisory” (limitations and interpretation).
cdc.gov -
National Traffic Court Resource Center — “The Marijuana Detection Window” (review of detection windows at common cutoffs).
ntcrc.org (PDF) -
University of Rochester Medical Center — Cannabinoid screen and confirmation (urine) overview, including 50 ng/mL threshold discussion.
urmc.rochester.edu -
Testing.com — Marijuana (THC) testing overview; explains that cutoffs vary (20/50/100) and that many federal programs use 50 ng/mL screening and 15 ng/mL confirmation.
testing.com -
SAMHSA — National Helpline (24/7 treatment referral).
samhsa.gov