THC, the active compound in marijuana, is detectable in blood for a relatively short window: up to about 12 hours after your last use if you smoke or vape, and potentially a few hours longer if you consume edibles. That makes blood the shortest detection window of any standard drug test. But the full picture depends on what exactly the test is looking for, how often you use, and how you consumed it.
Detection Window for Blood Tests
When you inhale marijuana, THC concentrations in your blood peak within about 10 minutes. From there, levels drop quickly as THC moves out of the bloodstream and into fatty tissues, organs, and the brain. For most people, active THC becomes undetectable in blood within 3 to 12 hours after smoking or vaping.
Edibles follow a different timeline. Because THC has to pass through your digestive system and liver before reaching your bloodstream, peak blood levels are delayed by 2 to 4 hours after eating. The concentrations reached are also much lower than with smoking, since the liver breaks down most of the THC before it ever enters circulation. Oral bioavailability is estimated to be as low as 6%. This means edibles produce a slower rise and a more gradual decline in blood THC, but the overall detection window stays in a similar range.
What the Test Is Actually Measuring
This is where things get more complicated. Blood tests can look for two different things: active THC (delta-9-THC) or its inactive metabolite, THC-COOH. The distinction matters a lot.
Active THC is the compound responsible for the high. It clears from your blood quickly, which is why the detection window is measured in hours. THC-COOH, on the other hand, is what your body produces after the liver processes THC. It’s not psychoactive, but it lingers far longer. In chronic, daily users, THC-COOH can remain detectable in blood for up to a month after the last use. For occasional users, it clears much faster.
Most blood tests used in legal or workplace settings are looking for active THC, not the inactive metabolite. That’s the main reason blood testing has such a short detection window compared to urine testing. But if a test specifically screens for THC-COOH in blood, the window expands dramatically for heavy users.
How Your Body Processes THC
THC is fat-soluble, which is the key to understanding why it behaves the way it does. After entering your bloodstream, THC rapidly distributes into fatty tissues throughout the body. This is why blood levels drop so fast: the THC isn’t gone, it’s just moved into storage. Over the following days and weeks, small amounts slowly release back into the bloodstream and get processed by the liver.
The liver converts THC into over 100 different metabolites. The two main ones are 11-OH-THC (which is as potent as THC itself, or even more so) and THC-COOH (which is inactive). THC-COOH is ultimately excreted, with about 65% leaving through feces and 20% through urine. In frequent users, THC accumulates in fat tissue over time, which is why metabolites take longer to fully clear.
Blood vs. Urine vs. Other Tests
Blood tests have the narrowest detection window of any common drug screening method. Here’s how they compare:
- Blood: A few hours for active THC, potentially up to a month for inactive metabolites in heavy users
- Urine: Approximately 1 to 30 days depending on frequency of use, with single-use detection at the shorter end and daily use at the longer end
- Hair: Up to 90 days
- Saliva: Typically 24 to 72 hours
Because THC leaves the bloodstream so quickly, blood tests aren’t commonly used for workplace drug screening. Urine tests are far more practical for detecting past use. Blood tests are primarily used in situations where recent use or current impairment is the question, such as traffic stops or accident investigations.
Legal Thresholds for Blood THC
Several U.S. states have set specific blood THC limits for driving under the influence. Six states use a “per se” limit of either 2 or 5 nanograms per milliliter (ng/mL) of THC in blood, meaning you can be charged with impaired driving if your blood level exceeds that threshold, regardless of how you actually appear to be driving. Twelve additional states have zero-tolerance laws, where any detectable amount of THC in blood is enough for a charge.
These laws are controversial. Research from the Association for Diagnostics and Laboratory Medicine has noted that there is no established correlation between THC blood concentration and actual driving impairment. Unlike alcohol, where blood alcohol content reliably predicts impairment, THC levels in blood don’t map neatly onto how impaired someone is. Regular users can have measurable THC in their blood while completely sober, and occasional users might be significantly impaired at lower concentrations.
Factors That Affect How Long THC Stays in Blood
The 12-hour general estimate is just that: a general estimate. Several variables push your personal timeline shorter or longer.
Frequency of use is the biggest factor. If you use marijuana daily, THC accumulates in your fat stores and continuously re-enters your bloodstream at low levels. A single-use event clears much faster because there’s no backlog of stored THC. Body fat percentage plays a role too, since THC is stored in fat tissue. People with higher body fat may retain THC and its metabolites slightly longer.
Method of consumption also matters. Smoking and vaping produce a sharp spike in blood THC followed by a rapid decline. Edibles produce a lower, slower curve. The dose itself is relevant as well: a single puff versus multiple sessions will produce very different blood concentration profiles. Metabolism, hydration, and individual liver enzyme activity all introduce further variation, but these are harder to predict or control for.