A: Marijuana" does not enter your blood stream, THC the active element in cannabis does. THC can remain detectable in the body for up to thirty days, and in hair samples for up to six months. There is no reliably proven way to remove THC from your system, and the products sold by head shops and supplement stores are "snake oil," a scam. The only way to make it indetectable in hair would be to remove all body hair.