Commonly, I am asked how long marijuana stays in a person’s system. This is a bad question, because it is imprecise—by system, do you mean in a person’s urine, hair, or blood? If the concern is drug testing through probation in Denton and most counties, urine is the most common method of testing. Any substance will remain in hair longest. For blood testing, levels generally spike following use, and generally remain in the body for several days in easily detectable form.

So how long does marijuana stay in a person’s urine?

First, you need to understand how marijuana “gets into” your system, per se. Marijuana doesn’t stay in any bodily system; THC, specifically, does. THC, a metabolic byproduct of marijuana consumption, does stay in your system, and is precisely what is being tested for by urine screenings. THC is a lipid-soluble byproduct of marijuana (meaning it can enter a cell by dissolving the lipid component of a cell’s membrane.) Because people have different metabolic rates, how long THC stays in a person’s system can vary by person.

That said, different studies have been conducted examining how long THC remains in a person’s urine, using both chronic (defined as daily) smokers. With the huge caveat that metabolization of THC varies from person to person, most scholarly studies had chronic marijuana smokers peeing clean in 40 days, with an average window of 13 days (meaning the average time from smoking to clean pee was 13 days, but some people had THC in their urine for 40 days.)

So, for habitual marijuana smokers, 40 days of not smoking marijuana or being exposed to marijuana smoke should be more than enough for a clean urine test. Likely, a clean urine test can be achieved in significantly less than 40 days—somewhere around 20 days.