Drugs in the Body
Medicines can enter the body in many different ways, including through an inhaler, a skin patch, a pill or a hypodermic needle. As drugs make their way through the body, many steps happen along the way. Understanding how medicines work in your body can help you learn why it is important to use medicines safely and effectively. In this section on taking medicines, we’ll focus on medicines you take by mouth, since those are the most common.
Entering and Circulating in the Body
When you take medicines by mouth, they move through the digestive tract and are taken up by internal organs like the stomach and small intestine. Often, they are then sent to the liver, where they might be chemically altered. Finally, they are released into the bloodstream.
As the bloodstream carries medicines throughout the body, the drugs can interact with many tissues and organs. Side effects can occur if a drug has unintended effects anywhere in the body.
Just as it does with food, the body tries to chemically break down medicines as soon as they enter the body. Most drugs taken by mouth enter the stomach or small intestine and then are sent to the liver.
The liver contains protein molecules called enzymes that chemically modify drugs and other non-food substances. The chemical alteration of a medicine by the body is called drug metabolism.
Often, when a drug is metabolized by the body, it is converted into products called metabolites. Usually, these metabolites are not as strong as the original drug. But in some cases, they can have effects that are stronger than the original drug. For example, codeine in the prescription pain killer Tylenol#3 becomes fully active only after the medicine is metabolized in the liver.
Because most drugs and other “foreign” substances are broken down in the liver, scientists refer to the liver as a "detoxifying" organ. As such, the liver can be prone to damage caused by too much medicine in the body.
Drug metabolites often return to the liver and are chemically altered once again before they exit the body.
Exiting the Body
After a drug’s metabolites have circulated in the bloodstream, where they work as medicine, the body eliminates them the same way it eliminates other wastes—in the urine or feces. Age-related changes in kidney function can have significant effects on how fast a drug is eliminated from the body.