How Long Does Marijuana Stay in Your System? Answering the Question "How long marijuana stays in your system" is an interesting question, and a very popular one on the Internet. Answering it is trickier than you might believe, but the answer is important if you are administering a drug test.
The first important distinction to make is that even if someone is no longer experiencing the effects, or "high," of marijuana, it is still in their system. And it is not so much marijuana that is in your system, but its active chemical, THC (Tetrahydrocannabinol).
Drug tests look for THC by analyzing saliva, hair, or urine, and sometimes blood. The variation in how long THC can be detected in a standard saliva, hair, or urine consumer drug test might surprise you.
In the case of saliva, marijuana escapes the system quickly and takes longer than most drugs to be detected. About an hour after use, marijuana can be detected by a home saliva drug test. It takes about 12 hours for marijuana to begin leaving saliva - after that time, the accuracy of a saliva-based home drug test drops greatly.
When it comes to urine drug testing, the amount and frequency of real weed for sale smoked by the user makes a huge difference in how long THC can be detected. Marijuana can stay in your urine for up to 45 days; somewhere between a week and 30 days is more typical. Someone who uses marijuana frequently or daily can expect to be positive on a drug test for at least three weeks or more after giving up the habit. And one-time users should not expect to get a negative drug test result just because they only tried the drug once - a urine drug test will still find them positive from a few days to over a week after use. Marijuana use can generally be detected from 2-5 hours after initial use in the urine.
Although marijuana/THC may have left the saliva and urine, it can still stay in your system - in your hair follicles. When you consume a drug, it enters the bloodstream. Blood nourishes your hair and helps it to grow, and if the blood has any drug traces in it, they become trapped in the hair and grow out along with it. After about a week of drug use, the hair will grow out far enough from the scalp to be cut, and drug
use can be detected in that hair. Drugs take a very long time to deteriorate in hair - consumer hair testing labs will only go back 90 days (industry standard) but courts and so forth can mandate a test that analyzes the hair over a longer period. Naturally, your body hair will also contain drug traces, and since body hair grows more slowly, the detection window for body hair is considered to be a year. The only caveat is that with hair drug testing, marijuana does not always incorporate well with the hair, so even a frequent user may test negative (while someone who smokes rarely might test positive). It is unknown as of yet why this is the case, as this phenomenon does not occur with any of the other commonly hair tested drugs like cocaine. Basically, hair drug testing can detect marijuana in your system, but is less reliable than urine tests, which are 98% accurate.
One of the factors that most influences how long marijuana stays in your system is the frequency of use. THC is fat soluble, and when marijuana is smoked the fat cells in the body absorb the THC. Over a few days, the THC diffuses into the blood and your body becomes clean. However, if you continually smoke, more THC will be taken into your fat that can be let out into the blood, and thus THC (and therefore, evidence of marijuana use) will stay in your system much longer. In fact, the New Scientist, a scientific journal, documents (rare) cases of former heavy users who, experiencing significant weight loss, start excreting "old" previously stored THC into the blood as a result of so much fat being burned up.
So when you ask the question of how long marijuana stays in your system, it is important to note - what part of the system do you mean? If you are testing a child or an employee you suspect of long-term use, you will want to ask these questions. Since marijuana is so variable, it may prove most wise to simply purchase a drug test and find out if you or the person you want to test is indeed clean.