How Long Does Weed Stay In Your System? A Pharmacologist Explains

Whether for work or custody, some seemingly legal marijuana users need to beat a system that is at odds with itself. Here's what you need to know.

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Marijuana is not legal in the United States. It’s easy to forget the fact that whether you live in weed-happy Colorado or quick-to-prosecute Idaho, the federal government does not recognize a right to carry, ingest, or grow marijuana. There are consequences to this fact if you choose to smoke pot, responsibly or not. Federal employment is one obvious one. If you’re in a job that tests, you need to pass that test. Custody is another sticky situation for marijuana users. Divorced dads can lose custody rights of their children if they fail that test.

So what’s a responsible marijuana user to do? How long does marijuana stay in your system anyway? And is there a threshold for passing the test? Here’s what the science says.

How Long Marijuana Stays In Your System

After you use marijuana, your body has to break down THC, the active ingredient. THC gets into the bloodstream and is temporarily stored in fat and organs, according to Healthline. Long after you come down from your high, the liver has to process the THC and break it into metabolites, which is what drug tests detect. Because THC binds to fat, it takes longer for traces of marijuana to leave the body than other drugs such as alcohol.

This is how long THC metabolites can be detected in different parts of the body, according to Medical News Today:

  • Blood: 3-4 hours
  • Saliva: 1-3 days
  • Urine: 3-30 days
  • Hair: Up to 90 days

The exact amount of time it takes for marijuana to leave your system varies from person to person. Age, sex, and BMI all affect how fast you can metabolize THC. Chronic use, higher doses, more potent strains, and eating edibles rather than smoking marijuana can all increase the length of time it’s detectable in your body.

How Marijuana Drug Tests Work

The most common drug test for marijuana is the urine test because it’s easiest to access, says Namandje Bumpus, Ph.D., previously a professor of pharmacology and molecular sciences at Johns Hopkins Medicine and now Chief Scientist at the FDA. Like with many drug tests, you pee in a cup, and lab techs run it for a particular metabolite of THC. The test can run positive for up to three days if you smoked once, five days if you smoke four times per week, 10 days for daily users, and 30 days for chronic heavy users, according to Mayo Clinic Laboratories.

Blood tests are less common because they can only detect THC for several hours after use. Saliva tests also test for THC itself and are primarily used by police officers and others who are looking for signs of recent use.

Hair tests are rare but can get a sense of whether a person has used marijuana over a greater length of time. Remnants of weed reach the hair through tiny blood vessels.

If a person tests positive for marijuana, a second confirmatory test is usually performed to rule out a false-positive result.

How to Flush Weed Out of Your System

If you’re looking for a quick fix to rid your body of weed, unfortunately, there is none. “As for the metabolism, it’s not something you could reliably speed up,” Bumpus says.

Some people suggest exercising to speed up metabolism of THC, but it probably doesn’t do much. “The rate of metabolism is really going to be dependent on proteins in your liver and how fast they’re acting,” she says.

Drinking water can help your kidneys excrete the metabolites, Bumpus adds, but it won’t speed up metabolism. Some people drink a lot of water before getting their urine test to dilute their sample and reduce the risk of THC metabolites being above the cutoff level. However, drinking too much water can get your sample flagged for purposeful dilution, and you may need to provide another.

If You Can’t Beat It, Fight It

If you have to take a drug test, you probably don’t have time to let marijuana clear from your system. That’s why it’s important to know your rights. You may be able to avoid getting screened for cannabis use. If you do test positive, there may be legal action you can take to defend your employment or custody rights.

If you live in Arizona, Delaware, or Illinois, congratulations: You can’t be fired for using medical marijuana off the clock. If you live anywhere else, it’s up to your boss to decide.

There’s less protection for legal recreational use. Most states allow employers to screen potential employees for drugs, including marijuana, before hiring them and to deny them employment if the test comes back positive. However, not all states allow random testing of current employees. In many cases, there must be a reason to test someone who legally uses pot, such as concern that the employee’s use is harming others. If you think you were given a drug test unfairly, you may be able to argue your case in court.

If the drug test is used as evidence in custody arrangements after a divorce in states where marijuana is legal, it all comes down to the judge’s discretion. Proof of marijuana use alone may not be enough to lose rights, and there may also need to be proof of harm or risk to the child.

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