The effects of smoking marijuana fade quickly, but the drug can be detected in the body for weeks and sometimes longer. How long it remains in the system depends on how often or how much marijuana the user has been smoking. How Marijuana Is Metabolized The active ingredient in marijuana -- Tetrahydrocannabinol or delta-9-THC or simply THC -- enters the body's bloodstream rapidly after smoking marijuana. If marijuana is ingested, rather than smoked, it takes longer to be absorbed into the blood, usually from 20 minutes to an hour and a half. But THC is detectable in the blood for a short time, usually a few hours, because it is rapidly metabolized into molecules known as metabolites. At least 80 different metabolites are formed from THC. These metabolites are stored in body fat and are gradually eliminated from the body through feces and urine.