Most drug testing is broken down into two categories, urine testing or urinalysis and hair testing. Once the body takes in a substance, the end product looks much different, once it passes through.
Alcohol goes in as beer, gets oxidized by the liver, and comes out as water.
Drugs go in the body in the psychedelic form and come out as a metabolite. This is oxidized by the body and comes out as 31 different metabolites. Marijuana is passed from the lungs to the blood stream, and while the drug is freely floating in the body in sufficient quantity a high is felt. Marijuana is liposoluble, meaning it absorbs into fat cells. The drug is stored in the fat cells indefinitely untill the body burns the fat cells for energy.