There is no single national legal limit for cannabis on a drug test. A handful of states set THC thresholds for driving, most commonly 5 ng/mL in whole blood, while workplace tests follow federal urine cutoffs of 50 ng/mL for screening and 15 ng/mL for confirmation. A positive result is not proof of impairment, and living in a legal state does not make a test result legal.
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Is there a legal limit for THC on a drug test?
No federal limit exists for cannabis, unlike the 0.08% blood alcohol standard. Cannabis remains a controlled substance under federal law, so federal agencies and DOT regulated employers treat any confirmed positive as a violation. State laws vary widely, and they answer different questions: some govern driving, some govern employers, and none of them govern a home test.
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What blood THC levels do states use for driving?
About a dozen states have set a numeric threshold for THC or its metabolites. The most common figure is 5 ng/mL of delta-9 THC in whole blood, and several states still prosecute on any detectable metabolite.
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- Pennsylvania: 5 ng/mL THC in blood.
- Montana: 5 ng/mL THC in blood.
- Nevada: 2 ng/mL THC in blood, among the lowest in the country.
- Colorado: 5 ng/mL creates a permissible inference of impairment, not automatic guilt.
- Illinois: 5 ng/mL THC in blood or 10 ng/mL metabolite in urine, also a permissive inference.
- Washington: 5 ng/mL, since softened from automatic proof to a rebuttable inference.
What cutoffs do employers and labs use?
Most workplace panels mirror the federal guidelines, which are written for urine. A sample must screen positive at 50 ng/mL and then confirm at 15 ng/mL of the metabolite THC-COOH before it is reported as a positive. Oral fluid and hair tests use far lower cutoffs, sometimes in the single digits of ng/mL or around 1 pg/mg for hair.
Why doesn't a number prove impairment?
THC is fat soluble and clears slowly, so blood levels lag behind actual effects. Daily users can sit above a threshold while completely sober, and occasional users can be impaired at a low reading. Federal traffic safety researchers have found no THC concentration that reliably predicts impairment across all drivers.
How long does cannabis stay detectable?
Detection windows depend on the test, frequency of use, body fat, and dose. Urine is the longest common method, blood and saliva show recent use over hours to a couple of days, and hair can hold a signal for months.
- Urine: up to 3 days after one use, 30 days or more in heavy daily users.
- Blood: roughly 1 to 2 days for chronic users, hours for casual use.
- Saliva: typically 24 to 72 hours.
- Hair: up to 90 days per half inch of length.
Can legal use still cost you a job?
Yes, in most of the country. Federal contractors, DOT regulated roles, safety sensitive positions, and many private employers can test and decline to hire on a positive result. A medical card can explain a result but does not override federal rules or an employer policy.
Which states protect off duty use?
A growing group of states limits employer testing for off the clock use. California bars employers from discriminating based on off duty cannabis use and from testing for non psychoactive metabolites, with carve outs for federal contractors and construction. New York, Minnesota, and Nevada have similar protections with their own exceptions.
Quick answers
Does CBD cause a positive test?
Full spectrum CBD can carry enough THC to trigger a positive, especially with heavy daily use. Isolate products are lower risk, but not zero risk.
Can secondhand smoke fail a test?
Rarely, and usually only after heavy exposure in an enclosed space. Most labs confirm positives at a level casual exposure will not reach.