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:
- an alteration in blood flow to the intestine;
- change in drug metabolism (breakdown) by the intestine;
- increased or decreased intestinal motility (movement);
- alterations in stomach acidity, and
- a change in the bacteria that normally reside in 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 (for example, food) binds to the drug and prevents its absorption.
Change in drug metabolism and elimination
Most drugs are eliminated through the kidney in either an unchanged form or metabolized by the liver. Therefore, the kidney and the liver are very important sites of potential drug interactions. Some drugs are able to reduce or increase the metabolism of other drugs by the liver or their elimination by the kidney.
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