THC is typically detectable in blood for 2 to 12 hours after a single use, though it can show up for as long as 24 hours in some cases. For heavy, daily users, the picture changes dramatically: THC’s half-life in plasma extends to roughly 4 days, meaning traces can linger in the bloodstream for a week or more after the last use.
Detection Windows for Occasional vs. Heavy Use
If you’ve used cannabis once or only occasionally, blood tests will generally pick up THC within the first 2 to 12 hours. After that window closes, active THC drops below most standard lab cutoffs. Some sources extend this to 24 hours depending on the dose and individual factors, but for a one-time use, the next day is roughly the outer boundary.
Chronic users face a much longer timeline. When researchers studied daily cannabis smokers, they estimated THC’s plasma half-life at about 4.1 days. A half-life means the concentration drops by half in that period, so it takes several half-lives for levels to become undetectable. In practice, a daily user who stops completely could still have measurable THC in blood for one to two weeks, and possibly longer depending on how much they were consuming.
The reason for this difference comes down to fat storage. THC dissolves easily into body fat. With occasional use, there isn’t much stored, so what enters your blood clears quickly. With repeated use, THC accumulates in fatty tissue and slowly releases back into the bloodstream over days, creating a reservoir that keeps blood levels elevated well after you stop.
Active THC vs. Its Metabolites
Blood tests can look for two different things: active THC (delta-9-THC) and its breakdown products, particularly an inactive metabolite called THC-COOH. Active THC disappears from blood relatively quickly. THC-COOH sticks around much longer because the body eliminates it at a slower, more variable rate.
This distinction matters because active THC correlates more closely with recent use, while the metabolite simply confirms that cannabis was consumed at some point. Some drug tests measure both. If your concern is a roadside or workplace blood draw, knowing which compound is being tested changes what “detection window” actually means for you. The metabolite can be found in blood days after active THC has cleared, especially in regular users with higher body fat or slower metabolisms.
How Consumption Method Affects Blood Levels
Smoking or vaping cannabis sends THC into your bloodstream almost immediately, with peak blood concentrations hitting within about 30 minutes. From that peak, levels drop steeply in the first few hours as THC redistributes into tissues. This sharp rise and fall is why blood tests are best at catching very recent inhalation.
Edibles follow a completely different curve. You won’t feel the effects for 30 minutes to 2 hours after eating, and the full peak may not arrive for up to 4 hours. The total duration of effects can stretch to 12 hours, with residual effects lasting up to 24 hours. Because THC from edibles is absorbed through the gut and processed by the liver before entering general circulation, blood levels rise more slowly, peak later, and can remain elevated for a longer stretch compared to the same dose inhaled.
Factors That Speed Up or Slow Down Clearance
Body composition plays a significant role. People with more body fat tend to store more THC and release it back into the blood more slowly. A lean person with a fast metabolism will generally clear THC faster than someone with a higher body fat percentage, all else being equal.
Genetics also matter more than most people realize. About one in four people carry a gene variant that causes their liver enzymes to break down THC less efficiently. For these individuals, the same dose produces stronger effects that last longer, and clearance from the blood takes more time. You can’t test for this at home, but if cannabis has always seemed to hit you harder or last longer than it does for friends using the same amount, slower metabolism may be part of the explanation.
Other variables include dose (higher doses take longer to clear), potency of the product, hydration, and overall liver function. None of these can be reliably manipulated to “flush” THC faster. Drinking extra water, exercising, or using detox products won’t meaningfully accelerate the rate at which your liver processes THC out of your blood.
Legal Thresholds for Blood THC
Six U.S. states currently enforce per se limits for THC in blood while driving, set at either 2 or 5 nanograms per milliliter (ng/mL). Colorado, for example, uses a 5 ng/mL threshold: if a blood draw shows THC at or above that level, it’s legally permissible to presume impairment. Nevada sets its limit at 2 ng/mL for THC, with a separate 5 ng/mL cutoff for the THC metabolite. Twelve additional states have zero-tolerance laws, meaning any detectable amount of THC in blood is enough for a charge.
These thresholds are controversial. Research has not established a reliable correlation between a specific blood THC concentration and actual driving impairment the way it has for blood alcohol levels. A daily user might sit above 5 ng/mL hours after their last use without feeling impaired, while an occasional user could be significantly affected at lower concentrations. The legal limits exist as bright-line rules, not as precise measures of intoxication.
For practical purposes, if you’re an occasional user and it’s been more than 12 to 24 hours since you last consumed cannabis, your blood THC is very likely below any legal threshold. If you use daily and recently stopped, you could remain above detectable limits for several days or longer, even without feeling any effects.