Drug Interactions

Whenever two or more drugs are taken concurrently, there is a chance that there will be an interaction among the drugs. The interaction may increase or decrease the effectiveness and/or the side effects of the drugs. It also may result in a new side effect, that is, a side effect not seen with the use of any one drug alone. The likelihood of drug interactions increases as the number of drugs being taken by a patient increases.

Therefore, people who take several medications are at the greatest risk for interactions. Drug interactions contribute to the cost of healthcare because of the medical care costs that are required to treat them. Interactions can also lead to pain and suffering that could be avoided. This month's topic discusses the issue of drug interactions and several ways to avoid them.

What are drug interactions?

A drug interaction can be defined as an interaction between a drug and another substance that prevents the drug from performing as expected. This definition applies to interactions of drugs with other drugs (drug-drug interactions), as well as drugs with food (drug-food interactions) and other substances.

How do drug interactions occur?

There are several mechanisms by which drugs interact with other drugs, food, and other substances. An interaction can result when there is an increase or decrease in: (1) the absorption of a drug into the body; (2) distribution of the drug in the body; (3) changes made to the drug by the body (metabolism); and (4) elimination of the drug from the body. Most of the important drug interactions result from a change in the absorption, metabolism, or elimination of a drug. Drug interactions also may occur when two drugs that have similar (additive) effects or opposite (canceling) effects on the body are administered together. Another source of drug interactions occurs when one drug alters the concentration of a substance that is normally present in the body. The alteration of this substance reduces or enhances the effect of another drug that is being taken. The drug interaction between warfarin (Coumadin) and vitamin K-containing products is a good example of this type of interaction. Warfarin acts by reducing the concentration of the active form of vitamin K in the body. Therefore, when vitamin K is taken, it reduces the effect of warfarin.

Change in absorption

Most drugs are absorbed into the blood and then travel to their site of action. Most drug interactions that are due to altered absorption occur in the intestine. There are various potential mechanisms through which the absorption of drugs can be reduced. These mechanisms include an alteration in blood flow to the intestine, metabolism (alteration of the drug) by the intestine, increased or decreased intestinal motility (movement) within the intestine, alterations in acidity in the stomach, and a change in the bacteria of the intestine. Drug absorption also can be affected if the drug's ability to dissolve (solubility) is changed by another drug, or if a substance (e.g., food) binds to the drug and prevents its absorption.