Regardless of whether you smoke a little pot or a lot, since THC is stored in fat-soluble cells in the body, the way it is processed is different for every person. So if you're wondering how long weed stays in your system, it depends on things like how often you smoke or eat edibles, your weight and exercise level, as well as other lifestyle factors. (Joe Amon, Denver Post file)

How long does weed stay in your system?

A surprise request for a workplace drug test has you choked and sweating and asking yourself how long weed stays in your system. What remains in your blood or urine from your bedtime tokes, or edible-fueled fun on the weekends? Are you going to get caught?

Here’s a short rundown on what you need to know, to keep your head straight.

At this time, there is no simple answer to the question. That’s because THC, the psychoactive element in marijuana, is fat-soluble, not water-soluble. If THC was water-soluble, all traces would be gone from the body within days.

That’s not the case here! To get a solid medical and research-based perspective, I asked two experts for their input via email. Dr. Alan Shackelford has professional experience that includes serving as a clinical and research fellow at Harvard Medical School, and Paul Armentano is deputy director of the National Organization for the Reform of Marijuana Laws and the NORML Foundation, as well as the 2013 winner of the Alfred R. Lindesmith Award for Achievement in Scholarship, given by the Drug Policy Alliance.

Lifestyle factors, including as a person’s consumption frequency, are variables that affect how long marijuana stays in the system. And Shackelford says it gets more complicated.

THC in the body

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“Unfortunately, there is no short or easy answer to that question. The cannabinoids are fat-soluble and are stored in body fat and released from there over time, so whether they show up on drug testing (which I assume is the actual question) depends on many factors, including how much cannabis has been used over what period of time, how much body fat someone has, exercise patterns, diet and others.”

To measure THC in the body, drug screenings actually measure carboxy-THC, a breakdown metabolite of marijuana. And that process is also affected by a number of factors. Says Armentano: “The body’s excretion of carboxy-THC is influenced by the subject’s metabolic rate, percentage of body fat, and is also likely moderated by stress levels and diet.”

Even a halt in marijuana use doesn’t stop the peak of THC levels in the body. THC levels can actually increase for days after someone ceases marijuana use. Armentano notes that “carboxy-THC levels may also spike on subsequent days, irrespective of new cannabis use.”

Drug screenings have a threshold for measuring THC levels in the body. Shackelford says, “In general, amounts of THC above 50 ng/mL (nanograms per mililiter) in urine are reported as positive, while amounts below 50 ng/mL are reported as negative. How much THC would result in a 50 ng level is impossible to predict.”

So, there’s no certainty in gauging how long THC metabolites will linger in your system after a weekend of partying or what THC metabolite levels for regular cannabis users might be.

Shackelford  summarizes: “Unlike alcohol, there is no easy or relatively reliable way to predict how quickly THC will be metabolized or no longer be detectable in blood or urine.”

Armentano cites a study that says THC stays in the body up to 100 days afterward.

So that’s the short answer: It could take more than three months before a person has cleared evidence of cannabis use from his or her system.