Arthritis Self-Management

Drugs and the Aging Body

by Stephen M. Setter, PharmD, CDE, CGP, FASCP, and Lindy Wood, PharmD

Your knees are aching, so you reach for the bottle of acetaminophen (Tylenol) or ibuprofen (Advil, Motrin), pop a tablet, and wash it down with some water. Or maybe you have high blood pressure, so every morning you take a prescription medicine to lower it. Easy enough, right? But what happens to that medicine once you take it — where does it go in the body and how does it start to work? Is the process the same in a young person and a senior? Does the fact that older people often take several drugs complicate matters? Here are some answers to these questions.

A drug’s path

When you swallow a pill, the most common way to take a medicine, it travels down your esophagus and into your stomach. There it mixes with the contents of the stomach and the stomach’s juices. The pill then begins to dissolve into smaller particles, and the stomach pushes these particles, and any other food or drink you’ve been digesting, into the small intestines. It is within the intestines that most medicines are absorbed into the bloodstream. But they aren’t cleared for delivery to their target quite yet.

Before the tiny particles now in the bloodstream can travel throughout your body and take effect, they must pass through the body’s “gatekeeper,” the liver. Almost all blood from the digestive system travels first to the liver to be inspected. The liver’s function is to filter and detoxify the blood. The blood coming from the intestines may have in it nutrients from food or particles of medicine. On rare occasions, there may even be toxins swallowed by accident. Before all these substances can reach the rest of the body, the liver filters the blood and decides what can pass, allowing harmless substances like nutrients to go on but stopping dangerous toxins.

As the liver examines your blood, it detects a foreign substance: your medicine. Its natural defense mechanism in response to this unfamiliar chemical is to activate its workforce of enzymes to “chew up” some of the medicine and filter it out of the blood. In trying to protect the rest of your body from something it considers a potential danger, the liver essentially “throws away” some of your medicine. This is one reason why drugs are given in so many different strengths. For some drugs, the amount of drug the liver removes is significant enough that the dose has to be higher to compensate. Otherwise there would be no drug left to reach the rest of the body and do its work. In fact, some drugs cannot be taken orally (by mouth) because the liver removes all the active medicine, even at high doses, leaving none to circulate through the body. These types of medicines are usually given in another form so they can avoid the liver — for example, as an injection or in a patch or cream so the medicine can be absorbed through the skin.

Besides acting as a filter, the liver has another interesting function that can be very useful. Many medicines work properly in the body only if the liver activates them. The same enzymes in the liver that “chew up” some drugs for removal activate other drugs and make them more effective.

Once the liver has screened the blood, the blood carries to other parts of the body the medicine that wasn’t filtered out. This way the medicine can reach its target, whether it is going to the heart and blood vessels to lower blood pressure, to the brain to help with memory, or to the joints to reduce pain and inflammation. Some medicines stay mainly in the blood to do their work. Others spread out into the muscles, joints, and fat.

After the drug has had its effect, the body must process what’s left of it for removal. The two main organs responsible for removing it are the liver and kidneys. With each pump of the heart, the liver and kidneys receive a large supply of fresh blood to filter. Together, the liver and kidneys receive about half of the blood from each heartbeat, much more than any other organ in your body. With each pass through the liver, some of the drug in the bloodstream is processed and removed, just as some of it was “chewed up” with liver enzymes before it started circulating in the bloodstream. Often, your liver does the legwork by breaking a medicine into smaller chemical structures that are then filtered through your kidneys into the urine for removal. As the drug is eliminated from the body, its effects begin to wear off, and the pain in your knees becomes noticeable again — it’s time for your next dose!

How age changes the body

As you can see, your body does a lot of work to process a medicine once you swallow it. Our bodies are very efficient at this, especially when we are healthy 25-year-olds. Fast forward several decades and it is likely that, among many other things, the way your body handles medicine has changed. The changes occur very gradually over the years, but they can greatly influence the way your medicine affects your body.

The throat. A drug doesn’t do much good if it doesn’t get down to the stomach and intestines, so the throat is the first hurdle it needs to deal with. One challenge some older adults face has to do with getting a pill down in the first place. Swallowing a pill can become tricky because of advancing age or a number of different medical conditions, including Parkinson disease, multiple sclerosis, or a stroke. A dry mouth, whether caused by dehydration, a medical condition, or even a medicine, can also make swallowing a pill more challenging. If swallowing is an issue, there are often alternate formulations of medicines that are easier to take, such as liquids or fast-dissolving tablets.

Last Reviewed on August 18, 2010

Stephen M. Setter is an Associate Professor of Pharmacotherapy at Washington State University College of Pharmacy. Lindy Wood is a Resident at the university.

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