It's got very little to do with metabolism or body fat and such shit. I'm 5'11" 170 lbs. and I do alot of heavey labor everyday at work (I work with metals) Very little body fat and a very high metabolism. I smoke every day about an ounce every 3 weeks by my self. I quit for 5 full weeks to pass a drug test for work and I still failed. Only fail safe way Iv'e found to pass a drug test is to clean out one of those travel size shampoo bottles and have someone (who's clean) piss in it for you. then keep it hidden in your scrotem. It hides well there and it keeps it at body temperature wich is essental for passing. You can tape one of those tempterature strips to the bottle to ensure proper temperature. You can get them at any pet store in the fish section. Also those pocket hand warmer sold in the sporting goods departments work well.