A lot happens in your body after you take a pill. Drugs that enter the body are transformed by myriad biochemical reactions that either break down the drugs into simpler substances or combine them (or parts of them) with natural body substances. This is the process of drug metabolism. Drug developers are attentive to the complexities of drug metabolism, because the way medications are processed in the body can make the difference between a drug that is safe and one that is ineffective or even harmful. Here's how it works.
Some portion of a drug may be lost as it passes through the gastrointestinal tract. Digestive enzymes in the stomach and the intestines can break down drugs, as can the actions of bacteria that normally live in the gut. Drugs can also interact with foods and beverages in the gastrointestinal tract, in some cases reducing, but in other cases increasing, the amount that gets absorbed.
After being absorbed through the gut, drug molecules travel via the portal vein to the liver. The liver is where most of the work of drug metabolism takes place. As the liver metabolizes drugs, it may produce chemical byproducts that are toxic to the liver itself. This is why taking too much of certain medications or taking them too often can harm the liver. The most common example of such a drug is acetaminophen (Tylenol).
In the liver, the work of drug metabolism is performed by enzymes (proteins that facilitate biochemical reactions). The liver's cytochrome P-450 system comprises more than two dozen chemically related enzymes that metabolize drugs.
Whatever remains of a medication after metabolism in the liver enters the hepatic vein, which carries blood from the liver to the heart. The heart then pumps the drug molecules out into the general circulation, which carries the drug throughout the body to its eventual target organ (and many other locations as well). Any molecules of the medicine that remain after traveling through the circulatory system eventually re-enter the liver via the hepatic artery, where the metabolic system can process them further.
The body excretes or eliminates water-soluble medications and their breakdown products (also known as metabolites) primarily in the kidneys, and the metabolites then pass out of the body through urination. Some medications or byproducts of drug metabolism that were handled by the liver pass back into the digestive tract through bile and later exit the body in the feces. Medications may also leave the body in saliva, sweat, exhaled air, and even in a mother's breast milk.
