Carboxy-THC is an inactive byproduct your body creates after breaking down THC, the psychoactive compound in cannabis. Its full chemical name is 11-nor-9-carboxy-delta-9-tetrahydrocannabinol (often shortened to THC-COOH), and it is the specific substance most urine drug tests are designed to detect. Unlike THC itself, carboxy-THC does not produce a high or any psychoactive effects. It simply lingers in your body as evidence that THC was processed at some point.
How Your Body Creates Carboxy-THC
When you consume cannabis, THC enters your bloodstream and travels to the liver, where enzymes begin transforming it in a two-step process. First, a liver enzyme called CYP2C9 converts THC into 11-hydroxy-THC (11-OH-THC), which is itself psychoactive and contributes to the high. Then, through a second round of oxidation by the same enzyme family, 11-OH-THC is converted into carboxy-THC. This final product has a carboxyl group (a cluster of carbon, hydrogen, and oxygen atoms) attached at carbon position 11, which is what distinguishes it chemically from active THC.
The key difference is that carboxy-THC does not cross into the brain effectively the way THC and 11-OH-THC do. That makes it pharmacologically inactive: it won’t affect your mood, coordination, or perception. It’s essentially metabolic waste, but waste that your body holds onto for a surprisingly long time.
Why Drug Tests Target Carboxy-THC
Active THC disappears from urine quickly, making it a poor marker for detecting past cannabis use. Carboxy-THC, on the other hand, persists in urine for days to weeks depending on how often someone uses cannabis. That long detection window is exactly why workplace and federally regulated urine drug tests use it as their target molecule.
Standard urine screening tests (immunoassays) are calibrated to flag samples at or above 50 ng/mL of carboxy-THC. If a sample triggers a positive at that threshold, a more precise confirmation test using gas chromatography-mass spectrometry is run with a lower cutoff of 15 ng/mL. These cutoffs are set by federal workplace testing guidelines and are designed to reduce false positives from very low-level or incidental exposure.
Oral fluid (saliva) testing works differently. Instead of looking for carboxy-THC, saliva tests target active THC itself, with an initial screen cutoff of 4 ng/mL and a confirmation cutoff of 2 ng/mL. This distinction matters because saliva testing reflects more recent use, while urine testing reflects accumulated metabolite over a longer period.
How Long It Stays in Your System
Carboxy-THC is detectable in blood plasma within minutes of smoking cannabis, and it remains measurable in plasma far longer than THC itself. In urine, detection windows vary dramatically based on frequency of use. A single or occasional use may produce a positive result for roughly 3 to 5 days. Regular use (several times per week) can extend detection to 1 to 3 weeks. For daily, heavy users, carboxy-THC can remain detectable for a month or longer after the last use.
These extended timelines exist because THC and its metabolites are fat-soluble. They bind to fat (triglyceride) molecules in your adipose tissue and accumulate there over time, especially with repeated use. As your body gradually breaks down fat stores, small amounts of THC and carboxy-THC are released back into the bloodstream, eventually making their way to the kidneys and into urine.
Fat Storage and Re-release
The relationship between body fat and carboxy-THC levels explains some patterns that catch people off guard. Research published in the British Journal of Pharmacology found that conditions triggering fat breakdown, such as fasting or stress hormone exposure, can measurably increase blood levels of both THC and carboxy-THC in subjects who had previously been exposed to THC but were no longer using it. In animal studies, 24 hours of food deprivation significantly raised blood THC-COOH concentrations compared to normally fed controls.
This means that periods of intense exercise, caloric restriction, or physiological stress could theoretically nudge stored cannabinoids back into circulation. For most people, the amounts released this way are small. But for chronic, heavy users with substantial accumulation in fat tissue, this re-release mechanism is one reason detection windows can stretch so long.
How Your Body Eliminates It
Within about five days of a single dose, 80 to 90% of THC-related compounds are excreted from the body. The majority, over 65%, leaves through feces rather than urine. Only about 20% is eliminated through urine. Before excretion, the liver attaches a molecule called glucuronic acid to carboxy-THC, making it more water-soluble so the kidneys can filter it out. This modified form, called THC-COOH-glucuronide, is the dominant version found in urine samples.
Lab technicians account for this by including a hydrolysis step during specimen preparation, which breaks the glucuronide bond and frees the carboxy-THC for accurate measurement. Without that step, the test would undercount the actual metabolite present.
Carboxy-THC and Impairment
One of the most important things to understand about carboxy-THC is that its presence does not indicate current impairment. Because it is pharmacologically inactive and can linger for weeks, a positive urine test tells you only that someone used cannabis at some point in the recent past. It says nothing about whether that person is high right now.
Researchers have attempted to correlate THC and carboxy-THC blood levels with measurable driving impairment, specifically lateral weaving on the road. While active THC in blood shows some relationship to impairment, carboxy-THC levels do not reliably predict it. This is a significant limitation in legal and workplace contexts: a positive carboxy-THC result can confirm exposure but cannot distinguish between someone who smoked an hour ago and someone who smoked two weeks ago.
Blood tests that measure the ratio of active THC to carboxy-THC can offer a rough estimate of how recently cannabis was used. A high ratio of THC to THC-COOH suggests recent consumption, while a low ratio (mostly carboxy-THC, very little active THC) points to use that occurred days or more in the past. Some forensic toxicologists use this ratio to provide more nuanced interpretations, though no universally accepted legal standard exists for it.