How Far Back Does a Mouth Swab Test Detect Weed in Your Body?

Sophia Delphi July 15, 2022 - 7 min read
Fact Checked
Illustration for how far back does an oral drug test detect weed

You’ve applied for a new job. Or you’ve already been hired, and the HR person is telling you what to expect at work. Or your probation officer is giving the lay of the land on what will happen going forward.

And you hear the words you’ve been dreading: “We drug test. For everything. Including marijuana.”

You’ve heard about testing from friends, or you’ve read about it online. So you know that weed shows up on some tests more often than it does on others.

There’s a question you desperately want to ask: “What type of testing do you do?”

That probably wouldn’t be smart, especially if you’re hoping to get hired. But if you think fast, maybe you can come up with something like “Awww. I hate giving urine samples – I’m never able to pee on command.” If you’re lucky, they’ll respond “Don’t worry, we do mouth swabs.”

Otherwise, you may have to find a co-worker or fellow probationer who can answer the all-important question.

Here’s why the type of test makes such a big difference: there’s a much shorter window during which an oral drug test detects weed.

Types of Drug Tests

Most employers and probation officers administer urine drug tests. That’s because they’re the cheapest option, they don’t have to be sent to a lab if the testing entity wants to save even more money, and they can often detect cannabis use dating back as far as 30 days.

Very few take blood or hair samples because those are the most invasive types of tests to conduct, they’re much more expensive to process, and they always have to be sent to laboratories for analysis. Hair provides the most information, sometimes showing drug use months before the test, but a blood test can’t detect weed use that occurred more than a few days in the past.

Relatively few employers, and virtually no probation departments, use saliva tests. There are two reasons why. Oral tests are the least reliable way to check for drugs, and they normally only come up positive for very recent use.

Why do some tests discover pot use going back weeks, while for others it’s only days? As Ron Burgundy would say, “it’s science.”

THC only stays in the body for a short time. Two-thirds of the cannabinoid is purged from the body within hours, and most of what’s left are broken down by the liver into metabolites like the substance called THC-COOH. Only a little remains in bodily fluids.

THC-COOH doesn’t disappear as quickly as THC; it’s stored in body fat, and the body eliminates it slowly over a period of weeks.

Blood and oral drug tests look for signs of THC, but urine tests detect the presence of THC-COOH. That’s why blood and saliva samples can only show recent weed use; the THC disappears in days.

Traces of THC-COOH, by contrast, show up in urine the entire time the metabolite is being purged from the body. It can still show up in urine weeks after smoking. (Hair is a different case; since it grows so slowly, traces of THC remain detectable for a long time.)

That’s why we said “you’re lucky” if you’re going to have to take a saliva test.

How Far Back Does An Oral Drug Test Show Weed Use?

Image of a Oral Drug Test

It depends primarily on two things: the last time you smoked, and how much/how often you smoke. (We’re using “smoke” as shorthand; tests can also detect vaping and edible consumption.)

Generally speaking, a saliva test can only detect THC for about three days. But if you’re an infrequent user, traces of THC may disappear within 24 hours. If you’re a very heavy smoker, or you prefer weed with high THC content, the THC may be detectable for a week after your last use; there have even been cases in which big-time stoners have come up dirty on an oral test weeks after they’ve (temporarily) abstained.

Needless to say, the best approach is to put the bud aside as soon as you know that a drug test is on the horizon.

Other factors can also affect how quickly or slowly THC will disappear from your system.

  • Those with active metabolisms, and those who are physically fit, flush THC from their bodies faster.
  • Those who are obese will keep THC in their system longer because it takes longer for the body to process and eliminates it.
  • Those who also use other drugs will retain the THC longer because their body will be trying to flush multiple toxins simultaneously.
  • Those who drink a lot of water will eliminate the THC from their body faster.

There’s one other reason why “you’re lucky” if you’re going to have to take an oral drug test. It’s easier to pass a saliva test on short notice.

How to Try Fooling an Oral Drug Test for Weed

Disclaimer: These suggestions are not guaranteed to work. They’ve worked for many people, but not for others. And if you’re a heavy smoker or prefer high-THC strains, trying to quickly eliminate signs of weed use may not be sufficient to test negative on a saliva test.

You undoubtedly know that an oral drug test involves swabbing the lining inside of your mouth. So it shouldn’t be surprising that the best way to trick this test is to clean the inside of your mouth as completely as possible.

Here are the steps to take.

  • Brush your teeth. Over and over again. You’re not going too far if you brush 3-5 times in the two hours before taking a saliva test. Don’t just brush your teeth, either; brush your gums and tongue carefully each time. Go the extra mile and buy a tongue scraper, too. (They don’t hurt as much as you’d think.)
  • Floss several times. Any little piece of food stuck in your teeth might contain THC residue.
  • Use a mouthwash after brushing. A disinfecting mouthwash is the best choice.
  • Don’t smoke tobacco for 48 hours. There’s a chance it could produce a false positive.
  • Drink lots of water in the days before the test. Never swallow the first gulp, though; swish it around in your mouth and spit it out.
  • Eat a lot of fatty foods in the days before your oral drug test. Fat absorbs THC, so red meat, chicken skin, butter, and ice cream – or if you’re the healthy type, avocado and fatty fish – may clear THC from your system more quickly.
  • We can’t vouch for them, but there are “saliva neutralizing gum” products on the market which supposedly also help when chewed shortly before the test.

None of this will ensure that you pass an oral drug test for weed; only abstaining for a week (or longer) before the swab will do that. But if you’ve smoked up within the few days before you’re tested, this method should at least give you a fighting chance of not coming up dirty.

How Far Back Does an Oral Drug Test Detect Weed FAQ

Q: Does a saliva test detect exposure to second-hand smoke?
A: We’d never say never, but it’s extraordinarily unlikely.

Q: Will this method work for oral tests for other types of drugs?
A: It should. And some drugs, like coke, disappear from the system even faster than THC.

Q: Will an oral test for weed detect the presence of CBD?
A: No. No drug tests look for signs of CBD use.