THC can stay in your system for several weeks, depending on a range of factors such as frequency of use, your individual metabolic rate, and body fat percentage. That said, THC detection times rarely exceed 30 days, according to a 2005 fact-sheet written Paul Cary, director of the Toxicology and Drug Monitoring Laboratory at the University of Missouri.
Tetrahydrocannabinol, or THC, the most notable of the active chemicals found in cannabis that is responsible for the plant’s well-known psychoactive effects, is fat soluble. That means THC binds to fat cells as opposed to water, which results in the cannabinoid taking significantly longer to be eliminated from your system than many other chemicals.
The method with which one consumes cannabis also affects which metabolic pathway THC follows in the body. That isn’t relevant only for the time it takes your body to excrete it, but also the time it takes for it to be metabolized and begin to affect you.
For instance, when cannabis is vaped or smoked, it is quickly absorbed by the lungs and there is a steep increase of THC in the bloodstream within minutes. The effects tend to last between one and four hours.
When cannabis is ingested, on the other hand, the THC enters the bloodstream through the stomach and intestinal walls and only then goes to the brain. The onset of THC’s psychoactive effects can take between 30-60 minutes and can last as long as six to eight hours, and sometimes longer. The psychoactive effects of edibles also tend to be felt more intensely due to a higher ratio of 11-hydroxy-THC (the active metabolite of THC) to delta-9-THC.
How long a THC high lasts and how long THC stays in your system are two very different things. There are some claims of “cannabis hangovers,” with anecdotal reports of lingering fatigue following cannabis use. However, in general, when the primary psychoactive effects of cannabis (e.g increased sensory awareness and euphoria) are over, things return to normal, despite lingering THC metabolites being stored in fat cells for several days or weeks.
Everyone Metabolizes THC Differently
After THC is consumed, it is quickly broken down and turned into molecules called metabolites. More than 100 THC metabolites have been identified and their half-lives differ greatly — some metabolites having a half-life of 20 hours, while others can be stored in fat tissue for days or weeks. Half-life, of course, is the time it takes for the concentration of a drug in the body to reduce by one half.
According to one report published in the Journal of Analytical Toxicology, the half-life of THC is 1.3 days on average for infrequent users, and up to 5-13 days for heavy users. Depending on the cutoff of the test being used, positive metabolites were still detectable between 4-12 days for infrequent users and up to 17-27 days in heavy users.
There are a range of factors which determine how long THC stays in your system.
- Metabolism: People have different metabolic rates due to age, activity levels, genetics, and health conditions. A faster metabolism results in a more rapid metabolization and excretion of THC.
- Body Fat: Since THC metabolites are stored in fat tissue, people with a higher body fat percentage will metabolize and excrete THC metabolites slower.
- Gender: Women have higher body fat percentages than men on average, and therefore will metabolize THC at a slower rate.
- Hydration: Dehydration results in more concentrates of THC metabolites to be present in the body, which may affect THC detection through urine testing methods.
There may not be a way to accurately predict how long THC will remain detectable. There is, however, enough clinical research out there that one could draw general estimates for how long THC might stay in the body.
Methods of Testing for THC
According to a 2004 review published in the Therapeutic Drug Monitoring journal, “detection time is longest in hair, followed by urine, sweat, oral fluid, and blood.”
Hair Follicle Testing
How long does marijuana stay in your hair? Hair follicle drug tests can assess previous drug use for up to 90 days. Cannabis reaches hair follicles through small blood vessels and so small traces of THC metabolites may remain in the hair. One study found that THC was detectable in 77% of chronic users, 39% of light users and 0% of non-users, indicating that THC metabolites detected in hair are strongly associated with frequency of use.
Urine testing is the most common type of THC testing. According to Mayo Clinic Proceedings, THC is detectable in urine for the following lengths of time:
- Single use: 3 days
- Moderate use (four times per week): 5-7 days
- Chronic use (daily): 10-15 days
- Chronic heavy use: Over 30 days
Sweat testing is a non-invasive cannabis testing method and is common in criminal justice and employment settings. In one study, THC elimination was evaluated in 11 daily cannabis users after stopping cannabis use. Sweat patches were worn for seven days, and were analyzed for THC. Eight out of the 11 subjects had negative results after the second week, while one subject produced positive results after four weeks. Although this sample size is small, these results do suggest that the majority of people will likely test positive for one week and negative results will likely follow in subsequent weeks.
- Occasional cannabis users: 1-3 days
- Chronic daily cannabis smokers: 28-29 days
Similar to other cannabis testing methods, the period of THC metabolite detectability in blood is dependant on frequency of use. According to a review published in the Therapeutic Drug Monitoring Journal, in general, THC is detectable in the blood for one to two days.
Other research has shown slightly longer THC blood detectability for heavy cannabis users. In a 2009 study, published in the journal Addiction, researchers tested THC blood concentrations in 25 heavy cannabis users after seven days of abstinence. Six out of 25 users had a positive test during the seven days, with six users having positive THC levels on the last day of abstinence.
Implications For Drug Testing
Millions of employees and potential employees are drug tested every year in the United States and elsewhere. They are measured for drug metabolites in their urine, blood, hair, saliva, breath, sweat, and even fingernails.
Many people attempting to fool test results frequently increase the amount of water they drink in order to “flush out” THC. The reasoning behind this approach is that over-hydrating oneself decreases the concentration of THC metabolites in the urine, decreasing the chances of positive results. It’s not unheard of for people to take vitamin B-12 to darken their urine in an attempt to fool testers who may be suspicious of extremely clear urine.
Excessive exercise in an attempt to burn fat, since THC is stored in fat, is not unheard of either. Ironically, a study published in the journal Drug & Alcohol Dependence shows that exercise releases stored THC metabolites into the bloodstream, making them more easily detectable. The bottom line is that there is no way to guarantee a negative test result for THC, and so there’s no method worth endorsing apart from abstinence if you expect to be tested.