Drug Interactions (cont.)

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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, such as supplements. Drugs also may interact with laboratory tests, changing the proper results of the laboratory test.

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 within the body;
  3. alterations 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. For example, there may be major sedation when two drugs that can cause sedation are taken simultaneously, such as narcotics with antihistamines.

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 effectiveness 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 ways that the absorption of drugs can be reduced. These mechanisms include:

  1. an alteration in blood flow to the intestine;
  2. change in drug metabolism (breakdown) by the intestine;
  3. increased or decreased intestinal motility (movement);
  4. alterations in stomach acidity, and
  5. a change in the bacteria that normally reside in the intestine.
Medically Reviewed by a Doctor on 1/30/2015

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