THC can stay in your system anywhere from a few hours to roughly 90 days, depending on the type of drug test and how often you use cannabis. A single use is typically undetectable in urine after about 3 days, while chronic heavy use can produce positive results for 30 days or longer.
The wide range exists because drug tests don’t all measure the same thing, and your body stores THC differently than most other substances. Here’s what actually determines how long it sticks around.
Detection Windows by Test Type
Each type of drug test has a different lookback period. The test you’re most likely to encounter, whether for employment or legal reasons, is a urine screen. But blood, saliva, and hair tests each tell a different story.
Urine Tests
Urine testing is the most common method, and it doesn’t actually detect THC itself. It detects an inactive byproduct your liver produces when it breaks THC down. That byproduct is fat-soluble, meaning it lingers in your body much longer than the high does. According to Mayo Clinic Laboratories, approximate detection times break down like this:
- Single use: up to 3 days
- Moderate use (about 4 times per week): up to 5 days
- Daily use: up to 10 days
- Chronic heavy use: up to 30 days, sometimes longer
Some sources cite detection in chronic users for up to 2 months in extreme cases. The standard urine test triggers a positive result when the concentration exceeds 50 nanograms per milliliter. After your last use, the metabolite typically shows up in urine within 60 minutes, though it can take up to 4 hours.
Blood Tests
Blood tests detect actual THC, not its metabolite, so the window is much shorter. THC is only detectable in blood for a few hours after use. This makes blood testing useful for determining very recent impairment, like in a traffic stop, but not for detecting use days or weeks earlier.
Saliva Tests
Oral fluid tests can detect cannabis for up to 24 hours after use. These are increasingly common for roadside testing and some workplace screenings because they’re quick and easy to administer. They’re designed to catch recent use rather than past use.
Hair Tests
Hair follicle testing has the longest lookback window: approximately 90 days. A standard sample of about 3.9 centimeters of head hair covers roughly three months of history. Hair tests are specifically designed to identify patterns of repeated use over time, so a single isolated use is less likely to show up than regular consumption. Quest Diagnostics notes that hair testing is the only method that provides this kind of extended usage history.
Why THC Lingers Longer Than Other Drugs
Most drugs dissolve in water, get processed by your kidneys, and leave your body relatively quickly. THC works differently. It’s fat-soluble, so your body stores it in fatty tissues of various organs. Once stored, it releases back into your bloodstream slowly over time, where your liver continues to break it down and your kidneys gradually excrete the byproducts.
This slow release is why detection windows are so much longer for frequent users. Each new dose adds to the THC already stored in your fat. A single use creates a small, quickly depleted reservoir. Months of daily use builds up a much larger one that takes significantly longer to clear.
Research from Johns Hopkins found that the urinary half-life of THC’s main metabolite is roughly 30 hours in a one-week observation period. That means every 30 hours or so, the concentration in your urine drops by half. But with longer monitoring, the effective half-life stretches to 44 to 60 hours, reflecting that deeper fat stores release THC more slowly over time. This is why the tail end of detection can drag on much longer than you’d expect from the initial drop.
Factors That Speed Up or Slow Down Clearance
Two people who use the same amount of cannabis can test positive for very different lengths of time. Several factors influence how quickly your body clears THC:
Body fat percentage is one of the biggest variables. Since THC is stored in fat tissue, people with higher body fat tend to retain it longer. This isn’t just about weight. Two people at the same weight can have very different body compositions, and the person with more fat tissue will generally clear THC more slowly.
Frequency and amount of use matters more than almost anything else. Occasional users clear THC quickly because there’s simply less of it stored. Chronic users have built up a significant backlog in their fat cells that takes weeks to fully release and metabolize.
Metabolism and physical activity also play a role. A faster metabolism processes THC more quickly. Exercise can theoretically mobilize fat stores and release THC back into the bloodstream, which is why some people worry about working out before a test. In practice, the effect of a single workout is small, but overall metabolic rate does influence your timeline.
Potency of the cannabis matters too. Higher-THC products deliver a larger dose, which means more THC gets stored and more metabolite needs to be excreted.
Can Secondhand Smoke Cause a Positive Test?
It’s unlikely under normal circumstances, but not impossible in extreme conditions. A study from Johns Hopkins placed nonsmokers in an enclosed, unventilated room while smokers went through 10 high-potency cannabis cigarettes. Some nonsmokers in that scenario had enough THC in their urine to trigger a positive on a standard drug test.
When the same experiment was run with ventilation fans on, the nonsmokers showed no meaningful effects beyond increased appetite. The study’s lead author described the unventilated setup as a “worst-case scenario” that couldn’t realistically happen to someone without them being fully aware of it. So while secondhand exposure is technically a factor, it would require prolonged time in a small, unventilated, smoke-filled space.
What the Tests Actually Measure
Understanding what’s being tested helps make sense of the timelines. THC itself, the compound responsible for the high, has a clearance half-life of under 30 minutes. It’s broken down so rapidly that it isn’t even detectable in urine. What urine tests pick up is the metabolite your liver produces, which is inactive (it doesn’t get you high) but extremely fat-soluble and slow to leave your body.
Blood and saliva tests, by contrast, measure the parent compound or its more immediate byproducts. That’s why their detection windows are measured in hours rather than weeks. They reflect recent exposure. Urine tests reflect cumulative exposure over days or weeks, which is why they’re the preferred method for most workplace and legal screening programs.
Hair testing works on yet another principle. As blood circulates through your scalp, trace amounts of drug metabolites get incorporated into the growing hair shaft. Since head hair grows at a fairly consistent rate of about 1.3 centimeters per month, a lab can estimate a roughly 90-day usage history from a single sample. This method is better at catching ongoing use than one-time exposure.