The part of marijuana that gets you high is called Delta-9-tetrahydrocannabinol. Most people just call this THC, but this is confusing: your body will change Delta-9-THC into more inert molecules known as metabolites, which don't get you high. Unfortunately, these chemicals also have the word 'tetrahydrocannabinol' in them and they are also called THC - so many people think that the metabolites get you high. Marijuana only keeps you high for a few hours, and it is not right to think that a person who fails a drug test is always high on drugs, either. THC is not water soluble which means that every time you use, it needs some place to stick to. That's where your fat cells come in. THC is stored in your fat cells. The more you have, the longer the THC will remain.

Depending on your history of us, 30 - 90 days later a test can still come up positive for THC. THC and its metabolites stay in the blood for days and in the fatty deposits of the body much longer. About 30% of THC that is ingested may remain in the body a full week after smoking. Some remnants from a single dose of THC may be detectable up to three weeks later. If you smoke say three to five times a week and have been smoking for three to five years then it can take upto 90 days before marijuna is undetectable in your system.