Medications and Drugs
Drug Interactions
Pharmacy Author: Omudhome Ogbru, PharmD
Medical and Pharmacy Editor: Jay W. Marks, MD
Drug interactions overview
Whenever two or more drugs are being taken, there is a chance that there will
be an interaction among the drugs. The interaction may increase or decrease the
effectiveness of the drugs or the side effects of the drugs. The likelihood of
drug interactions increases as the number of drugs being taken increases.
Therefore, people who take several drugs are at the greatest risk for
interactions. Drug interactions contribute to the cost of healthcare because of
the costs of medical care that are required to treat problems caused by changes
in effectiveness or side effects. Interactions also can lead to psychological
suffering that can be avoided. This review 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:
- the absorption of a drug into the body;
- distribution of the
drug within the body;
- alterations made to the drug by the body (metabolism);
and
- 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 have
sedation as side effects are given, for example, narcotics and 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 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;
- 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 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
as a by-product that results from the alteration (metabolism) of the drug 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.
Report Problems to the Food and Drug Administration
You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.
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