For a single use, THC typically clears your blood within 12 hours. For daily or heavy users, though, THC can remain detectable in blood for days, with the longest documented cases stretching to 30 days of abstinence. The wide range comes down to how much you use, how you consume it, and how your body stores and releases THC over time.
Detection Windows by Usage Level
If you smoke or vape cannabis once and haven’t used it recently, THC drops below detectable levels in blood within about 12 hours. That’s the straightforward case. Blood concentrations spike within minutes of inhaling, often reaching around 40 ng/mL after a moderate dose, then fall rapidly over the next few hours as THC moves out of the bloodstream and into your tissues.
For regular users, the timeline shifts dramatically. Multiple studies have found blood THC levels above 2 ng/mL persisting for up to 6 days after someone stops using cannabis entirely. In the most extreme documented cases, involving very heavy daily users, residual THC was still measurable in blood after 30 days of abstinence. This isn’t because the body keeps producing THC. It’s because THC stored in fat tissue slowly leaks back into the bloodstream over weeks.
Why THC Lingers in Heavy Users
THC is extremely fat-soluble. When it enters your bloodstream, 95% to 99% of it binds to fat-carrying proteins in your blood. Within minutes, it moves into highly active organs like your brain, lungs, and liver. Over the following hours, it redistributes more slowly into fat tissue, where blood flow is lower and the drug accumulates quietly.
Your liver breaks THC down through a series of steps. First, it converts THC into an active compound that still produces effects. That compound is then converted into an inactive byproduct called THC-COOH, which is further modified to become water-soluble enough for your body to excrete through urine and feces. The bottleneck in this whole process is the speed at which THC trickles back out of fat into the bloodstream for the liver to process. If you’ve been using cannabis daily for weeks or months, your fat tissue has built up a significant reservoir. Even after you stop, that reservoir keeps feeding small amounts of THC back into your blood for days or weeks.
This is why body composition matters. Someone with more body fat may retain THC longer than someone who is leaner, all else being equal. Metabolism speed, hydration, and physical activity also play roles, though none of these can dramatically accelerate clearance on their own.
Edibles vs. Smoking: Different Blood Profiles
How you consume cannabis changes both how quickly THC appears in your blood and how long it stays elevated. Inhaling cannabis, whether smoked or vaped, sends THC into your bloodstream within seconds. Blood levels peak within about 10 minutes and are substantially higher than what you’d see from eating the same amount of THC.
Edibles work differently. THC absorbed through your gut passes through the liver before reaching general circulation, and the liver metabolizes most of it on the first pass. Only about 6% of the THC in an edible actually makes it into your bloodstream as active THC. Peak blood levels are delayed by 2 to 4 hours and are much lower. After a 25 mg oral dose, peak THC in blood reaches roughly 3 ng/mL, compared to around 40 ng/mL after vaping 20 mg.
The tradeoff is that edibles produce a longer, flatter curve. THC stays in your blood at lower but more sustained levels, and the inactive metabolite THC-COOH actually reaches higher concentrations from edibles than from inhaling. After oral dosing, THC-COOH peaks at around 15 ng/mL compared to about 5 ng/mL after vaping. This matters because some blood tests measure that metabolite, not just active THC.
What Blood Tests Actually Measure
Not all blood tests look for the same thing. Some target active THC (delta-9-THC), which reflects recent use and correlates loosely with impairment. Others measure THC-COOH, the inactive metabolite, which sticks around in blood longer and indicates past use without saying much about current impairment.
Standard forensic laboratories using advanced testing methods can detect THC in blood at concentrations as low as 1.0 ng/mL. That’s a very sensitive threshold. At that cutoff, even trace amounts from fat redistribution in a heavy user could trigger a positive result days after last use. Workplace or medical blood tests may use different cutoffs, so the “how long” question partly depends on how sensitive the specific test is.
Legal Thresholds for Driving
If you’re asking about blood THC because of driving laws, the legal landscape is a patchwork. Eighteen U.S. states have zero-tolerance or specific per se limits for THC in blood. Ten of those states set the bar at zero, meaning any detectable THC in your blood is a violation regardless of impairment. Colorado uses a 5 ng/mL threshold as a “permissible inference” of impairment, meaning levels above that can be used as evidence against you, though you can still contest it.
The practical problem is that blood THC levels don’t map neatly onto impairment the way blood alcohol does. A chronic user might have 3 ng/mL in their blood the morning after use and feel completely sober, while an infrequent user at the same level might still be impaired. In zero-tolerance states, that distinction doesn’t matter legally. If THC is in your blood, you’re in violation.
Rough Timeline Summary
- Single use, inhaled: THC typically undetectable in blood within 12 hours
- Single use, edible: slightly longer detection due to slower absorption, but generally clears within 24 hours
- Regular use (several times per week): detectable for 2 to 7 days after stopping
- Heavy daily use over weeks or months: potentially detectable for up to 30 days, depending on body composition and test sensitivity
These ranges assume a standard lab cutoff of about 1 ng/mL. A less sensitive test with a higher cutoff would shorten these windows. Your individual timeline depends on how much THC your fat tissue has accumulated, how quickly your metabolism processes it, and what exactly the test is looking for.