One method of testing for alcohol is by drawing blood into 2 viles. The police department will send those samples to a lab where they will test the blood for alcohol. What happens with alcohol is that it gets absorbed into your bloodstream and then it goes to the brain affecting your judgment and reasoning. The samples will come back from the lab and will show the percentage of alcohol in your blood. Marijuana works the same way. When you smoke it, it gets absorbed into your bloodstream and goes to the brain. The difference between Marijuana and alcohol is that alcohol leaves the body MUCH quicker than alcohol.