Even 1 to 5 mg of THC Can Trigger a Positive Urine Test Weeks After Use, Study Finds

A new systematic review published in the journal Pharmacological Research finds that standard urine-testing cutoffs for marijuana can be exceeded after surprisingly small amounts of THC, especially when THC is taken orally, and that frequent users can stay above common workplace thresholds for weeks after stopping. Researchers from The University of Sydney reviewed 92 studies that measured urinary THC and THC-metabolite concentrations in people given marijuana products in controlled settings, as well as in people who used those products during ongoing use and during abstinence.

As noted by the review’s researchers, most urine tests do not look for THC itself. Instead, they target 11-nor-9-carboxy-Δ9-tetrahydrocannabinol, known as 11-COOH-THC, a terminal metabolite of THC that can remain detectable long after intoxication has passed. The review notes that urinary 11-COOH-THC is not a reliable indicator of impairment, even though it is widely used to determine whether a test is “positive.”

One key focus was how real-world use patterns affect the likelihood of testing positive under the most common thresholds. In workplace settings and other workplace-aligned contexts, the standard confirmatory cutoff is 15 ng/mL of total 11-COOH-THC. The researchers found that low single doses of THC in the 1.0 to 5.0 mg range, and very low repeated dosing below 1.0 mg per day, were sometimes enough to exceed that threshold, with oral ingestion appearing more likely to push results above the line.

The review also found that the timing issue is especially significant for people who use marijuana regularly. Weekly-to-daily users could remain above the 15 ng/mL confirmatory cutoff for weeks after stopping, meaning a positive test may reflect prior use rather than recent use, depending on the person and the circumstances.

The researchers also looked at competitive sports testing, where the World Anti-Doping Agency uses a higher Decision Limit: 180 ng/mL of total 11-COOH-THC. Even at that higher cutoff, the review found that low-to-moderate oral doses around 10 mg THC and moderate inhaled doses around 15 to 20 mg THC were sometimes enough to exceed the decision limit. Weekly marijuana use was also identified as a pattern that could sometimes produce results above that sports threshold.

While frequent users were more likely to remain above the lower workplace cutoff for extended periods, the review suggests many weekly-to-daily users often fell below the 180 ng/mL sports decision limit within about a week of stopping, though heavier users could take longer.

The review explains that outside of the two primary cutoff levels, different agencies may rely on different thresholds, and testing is often conducted in multiple steps. In workplace settings, for example, an initial screening test typically uses a higher cutoff, such as 50 ng/mL. If a sample is flagged, it then undergoes confirmatory testing using a lower cutoff, often 15 ng/mL. According to the review, criminal justice and rehabilitation programs generally follow similar standards to workplace testing, although some apply stricter screening thresholds.

The researchers say their synthesis is intended to help policymakers choose thresholds that match the behavior a program is trying to regulate, while also helping educate people about the risk of testing positive after different patterns of use. The study concludes by stating:

This systematic review synthesised the urinary THC and THC-metabolite concentrations reported in prior studies involving administration of cannabis/cannabis-based products and users of such products. Our synthesis contextualises the standard confirmatory cutoff used in workplace urine testing (and workplace-aligned contexts, e.g., criminal justice) (i.e., 15 ng/mL total 11-COOH-THC). It shows that this cutoff can be exceeded after low single doses (i.e., 1.0–5.0 mg) and very low repeated doses (i.e., <1.0 mg/day) of THC, particularly with oral ingestion, although the evidence supporting this is limited and inconsistent. It also demonstrates that weekly to daily cannabis users can remain above this threshold for weeks after cessation. Our synthesis further contextualises the WADA Decision Limit (i.e., 180 ng/mL total 11-COOH-THC). It shows that this limit can be exceeded after low-to-moderate oral doses (i.e., 10 mg) and moderate inhaled doses (i.e., 15–20 mg) of THC, as well as with weekly cannabis use. Weekly to daily cannabis users often fall below this threshold within a week of cessation. These findings provide an evidence base to support the selection of appropriate urine-testing thresholds and educate individuals about the risks of testing positive following different patterns of cannabis use.