THC, the active compound in marijuana, is typically detectable in a standard blood test for up to 12 hours after a single use. For heavy, daily users, the picture is very different: THC can remain detectable in blood for 7 days or longer after the last use. The gap between these two numbers comes down to how THC behaves in your body, particularly the way it hides in fat tissue and slowly leaks back into your bloodstream over time.
Detection Windows by Usage Pattern
If you smoke or vape marijuana once or only occasionally, THC peaks in your blood within about 10 to 14 minutes and then drops off quickly. For an occasional user, most standard blood tests won’t pick up THC after roughly 12 hours.
Chronic, daily users face a much longer window. A study of heavy cannabis users found that THC was still detectable in blood at least 7 days after their last use, even in a controlled setting where no new cannabis was consumed. This happens because THC is highly fat-soluble. Each time you use marijuana, some THC gets stored in your fat cells. With repeated use, those fat stores build up faster than your body can clear them. The compound then slowly diffuses back into the bloodstream over days or weeks, keeping levels above the detection threshold long after the high has worn off.
How THC Moves Through Your Blood
When you inhale marijuana, THC reaches peak blood concentration in under 15 minutes. After that initial spike, levels drop in two distinct phases. First, there’s a rapid decline as THC moves out of the blood and into tissues, including fat, brain, and organs. Then a much slower phase begins where levels taper gradually as THC is metabolized by the liver and excreted.
Edibles follow a different curve. Absorption is slower because THC has to pass through the digestive system and liver before reaching your blood. Peak concentrations are lower and more delayed compared to smoking. This means edibles produce a flatter, more drawn-out blood concentration curve, which can subtly affect how long THC stays measurable.
What Affects How Fast You Clear THC
Several factors influence how quickly THC leaves your blood:
- Frequency of use: This is the single biggest factor. The more often you use cannabis, the more THC accumulates in fat tissue, and the longer it takes to fully clear. A daily user for months will test positive far longer than someone who used once at a party.
- Body fat percentage: Since THC binds to fat, people with higher body fat may retain it longer. The compound has been detected in human fat biopsies 28 days after exposure.
- Metabolic rate: A faster metabolism breaks down THC more quickly, shortening the detection window. Age, activity level, and genetics all play into this.
- Dose: Higher doses mean more THC entering the system, which takes longer to process and eliminate.
Exercise Can Temporarily Raise Blood THC
Here’s a counterintuitive finding: exercise can actually cause a small, temporary spike in blood THC levels. When you burn fat during aerobic exercise, stored THC gets released back into circulation. A 2013 study confirmed that exercise produced a statistically significant increase in plasma THC levels in regular cannabis users. The increase was small, under 1 nanogram per milliliter, so it’s unlikely to push someone over a testing threshold on its own. But it does mean that a blood draw taken right after intense exercise could read slightly higher than one taken at rest.
Blood Tests vs. Other Testing Methods
Blood tests measure active THC, which means they’re best at capturing recent use rather than past exposure. This makes them different from urine tests, which look for THC-COOH, a non-psychoactive metabolite that lingers much longer. In the same study of heavy users, urinary metabolite concentrations exceeded 1,000 nanograms per milliliter more than five days after last use.
One complication with blood testing is that THC levels in blood don’t always correlate neatly with impairment. There’s a growing recognition among researchers that blood concentrations can be erratic and unpredictable, with a weak relationship between how much cannabis someone consumed and what shows up in their blood. For chronic users, residual THC from fat stores mixes with THC from recent use, making it hard to distinguish between “used yesterday” and “used an hour ago.” This is one reason blood THC testing for impairment remains controversial.
Legal Thresholds for Blood THC
Some states use blood THC levels to enforce impaired driving laws. Washington State, for example, set a legal limit of 5 nanograms of active THC per milliliter of whole blood. If a roadside test or hospital blood draw shows you at or above that level, it’s treated similarly to blowing over the alcohol limit. Colorado uses the same 5 ng/mL threshold. These per se limits apply regardless of whether you appear impaired, which is why understanding your own clearance timeline matters if you drive in a legal state.
For workplace or medical blood tests, the cutoff levels and testing protocols vary. Some labs test for active THC only, while others include metabolites. If you’re unsure what a specific test screens for, the ordering provider or lab can clarify what’s being measured and at what threshold.