While the effects of marijuana can be short-lived in one’s system, it can still be detected even after days or weeks. It all depends on how often one uses it and how much of it is used each time a person smokes it.
Marijuana contains the active ingredient tetrahydrocannabinol more commonly known as delta-9-THC or just plain THC. These compounds enter the system as soon as pot is smoked, but it takes a longer period of time to arrive in the blood stream if weed is ingested instead. The average time for its effect to manifest in the body if ingested is about 20 minutes to an hour.
Because THC is rapidly metabolized in the body to become metabolites, the substance can only be detected in the bloodstream for up to a few hours. THC is able to produce more than 80 types of metabolites which are stored in the body and gradually excreted through the urine or feces of its user.
Urine drug tests can detect THC and its metabolites for up to 13 days. There are some forms of metabolites, though, that can be easily eliminated with half-life rates from 20 hours up to 10 to 13 days usually stored in the fat tissues of the body.
Factors that affect how long marijuana stays in the body are the frequency and the amount of the illicit drug used as well as the length of time that a user has been consuming weed. If a pot smoker takes the drug on a daily basis, he can test positive for up to 45 days since the last date of use; heavy smoking of marijuana could render a user to test positive for up to 90 days after the last use.
False positive test results for marijuana testing are surprisingly rare unlike other common drugs. There isn’t any known substance that could trigger a positive result for marijuana especially for urine testing.