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Pharmacy Author:

Omudhome Ogbru, PharmD

Omudhome Ogbru, PharmD

Dr. Ogbru received his Doctorate in Pharmacy from the University of the Pacific School of Pharmacy in 1995. He completed a Pharmacy Practice Residency at the University of Arizona/University Medical Center in 1996. He was a Professor of Pharmacy Practice and a Regional Clerkship Coordinator for the University of the Pacific School of Pharmacy from 1996-99.

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Medical and Pharmacy Editor:

Jay W. Marks, MD

Jay W. Marks, MD

Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

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In this Article

• Drug interactions overview
• What are drug interactions?
• How do drug interactions occur?
• Change in absorption
• Change in drug metabolism and elimination
• What are the consequences of drug interactions?
• How often do drug interactions occur?
• How can drug interactions be avoided?
• Drug Interactions Index

What are the consequences of drug interactions?

Drug interactions may lead to an increase or decrease in the beneficial or the adverse effects of the given drugs. When a drug interaction increases the benefit of the administered drugs without increasing side effects, both drugs may be combined to increase the control of the condition that is being treated. For example, drugs that reduce blood pressure by different mechanisms may be combined because the blood pressure lowering effect achieved by both drugs may be better than with either drug alone.

The absorption of some drugs is increased by food. Therefore, these drugs are taken with food in order to increase their concentration in the body and, ultimately, their effect. Conversely, when a drug's absorption is reduced by food, the drug is taken on an empty stomach.

Drug interactions that are of greatest concern are those that reduce the desired effects or increase the adverse effects of the drugs. Drugs that reduce the absorption or increase the metabolism or elimination of other drugs tend to reduce the effects of the other drugs. This may lead to failure of therapy or warrant an increase in the dose of the affected drug. Conversely, drugs that increase absorption or reduce the elimination or metabolism of other drugs - increase the concentration of the other drugs in the body - and lead to increased amounts of drug in the body and more side effects. Sometimes, drugs interact because they produce similar side effects. Thus, when two drugs that produce similar side effects are combined, the frequency and severity of the side effect are increased.

How often do drug interactions occur?

The prescribing information for most drugs contains a list of potential drug interactions. Many of the listed interactions may be rare, minor, or only occur under specific conditions and may not be important. Drug interactions that cause important changes in the action of a drug are of greatest concern.

Drug interactions are complex and chiefly unpredictable. A known interaction may not occur in every individual. This can be explained because there are several factors that affect the likelihood that a known interaction will occur. These factors include differences among individuals in their:

• genes,
  
  
• physiology,
  
  
• age,
  
  
• lifestyle (diet, exercise),
  
  
• underlying diseases,
  
  
• drug doses,
  
  
• the duration of combined therapy, and
  
  
• the relative time of administration of the two substances. (Sometimes, interactions can be avoided if two drugs are taken at different times.)

Nevertheless, important drug interactions occur frequently and they add millions of dollars to the cost of health care. Moreover, many drugs have been withdrawn from the market because of their potential to interact with other drugs and cause serious health care problems.

How can drug interactions be avoided?

1. Give health care practitioners a complete list of all of the drugs that you are using or have used within the last few weeks. This should include over-the-counter medications, vitamins, food supplements, and herbal remedies.
   
   
2. Inform health care practitioners when medications are added or discontinued.
   
   
3. Inform health care practitioners about changes in lifestyle (for example, exercise, diet, alcohol
   
   
4. intake).
   
   
5. Ask your health care practitioners about the most serious or frequent drug interactions with the medications that you are taking.
   
   
6. Since the frequency of drug interactions increases with the number of medications, work with your health care practitioners to eliminate unnecessary medications.

This brief overview of drug interactions does not cover every possible scenario. Individuals should not be afraid to use their drugs because of the potential for drug interactions. Rather, they should use the information that is available to them to minimize the risk of such interactions and to improve the success of their therapy.

REFERENCES:

Ogbru, O. Drug-Food Interactions. Clinical Trends In Pharmacy Practice 1996;10:53-60

Ogbru O. Drug Interactions with Grapefruit Juice. Druglink 1997;1:59-60

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. Need help identifying pills and medications? Use the pill identifier tool on RxList.