When someone takes cocaine, they experience all its effects fairly quickly. Also, the body immediately goes to work destroying the drug. That’s what the body does. With all drugs. That’s why you have to take your blood pressure med everyday. If the body didn’t break it down, a single pill would last a lifetime—theoretically. But Mother Nature is too clever for that.
Drugs begin this metabolic break down almost as soon as the hit the bloodstream. In the case of cocaine, the breakdown products (called metabolites) include benzoylecgonine and methylecgonine. These products are then excreted by the kidneys and can be found in the user’s urine. But they are also excreted by the sweat glands—which are in many ways just a bunch of little kidneys—-so that these chemicals can often be found by toxicological examination of a deposited fingerprint.
This could prove useful in determining if a offender was under the influence, or had at least recently used, cocaine.
But it’s not just for cocaine or other illicit drugs. Such testing might be able to determine what medication you take and even what you had to eat.