Saliva tests for THC can produce negative results shortly after marijuana use while remaining positive for days among some frequent consumers, according to a new systematic review.
The study, published in the September 2026 issue of the International Journal of Legal Medicine, examined how long THC remained detectable in oral fluid using mass spectrometry and a cutoff of 5 nanograms per milliliter. That is the cutoff used for positive oral-fluid drug screening results under Australian workplace-testing standards.
Researchers reviewed 16 articles covering 37 experimental situations and 241 participants. The studies included frequent and occasional marijuana consumers, people exposed to secondhand smoke and participants who consumed THC through edibles, medicinal oils or other methods.
Two hours after marijuana use, 83% of frequent smokers remained above the cutoff, compared with 38% of occasional smokers and 59% of edible consumers. About 6% of those exposed only to secondhand marijuana smoke also tested positive at that point.
Estimated maximum detection periods varied substantially. The researchers calculated that approximately one in 100 passively exposed participants could remain positive after 3.3 hours. Among frequent consumers with the highest initial THC concentrations, the estimated maximum detection period reached 162 hours, or nearly seven days.
However, the researchers cautioned that the estimate for frequent consumers had an exceptionally wide confidence interval because relatively few tests were available for that portion of the analysis.
The review also identified the opposite problem: Some participants tested negative shortly after consuming THC, during a period when they could reasonably still be impaired. THC levels varied widely between individuals, even when participants consumed marijuana under controlled conditions.
The researchers said oral-fluid testing produced relatively consistent average detection periods at the population level but had “significant drawbacks on an individual level.”
They concluded that saliva testing may provide evidence of marijuana exposure, but a positive or negative result alone cannot reliably establish when someone consumed THC or whether that person was impaired at the time of testing.
Sponsors:
