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The 2021 edition of The Top 100 Drug Interactions includes several hundred new drug-drug interactions in 268 monographs. As in the past, we have included interactions that may produce clinically important adverse events based on their properties to assist you in predicting interactions were limited data are available. The table of CYP450 and drug transporter substrates and modifiers now includes more than 600 drugs. In addition, many of the existing monographs have been revised to reflect newly published data. Interaction entries are easy to find in the index by either trade or generic name. The Horn and Hansten Drug Interaction Probability Scale is included to assess the probability of a causal relationship between a potential drug interaction and a patient event. The Top 100 Drug Interactions is an authoritative drug interaction resource intended for all practitioners that fits in your pocket (8.5 x 5.5 inches).

View sample pages:
Warfarin interactions
Kinase Inhibitors
Cytochrome P450 Table

Drug-Drug Interactions

What are drug-drug interactions?

In the broadest sense, a drug interaction occurs whenever one drug affects the pharmacokinetics, pharmacodynamics, efficacy, or toxicity of another drug. The two drugs need not physically interact with each other to produce the effect. When the drug combination results in an undesired effect, the drug interaction becomes an adverse drug interaction. Drug interactions are much more common than adverse drug interactions. 

Should we be concerned about drug interactions?

 Drug-drug interactions are of potential concern whenever a person takes two or more medications concurrently. Indeed, in a recent poll adults were asked what they would be “very concerned” about if they were to check into a hospital or other health care facility. The number one concern (61%) was being given the wrong medicine, but a close second at 58% was a negative interaction between multiple drugs. 

How often do adverse drug interactions occur?

 It has been estimated that adverse drug outcomes occur about once every 100 patient days. These adverse events are costly and often preventable. Drug interactions produce asymptomatic alterations in drug response on a regular basis. Occasionally drug interactions result in obvious adverse outcomes. A study of 1000 older patients admitted to a hospital geriatrics unit identified 894 patients who were receiving two or more drugs. Although a majority of these patients were receiving potentially interacting drug combinations (60% of the patients), only about 15% of them had symptoms of an adverse drug interaction on admission. It is important to remember that these adverse drug interactions are predictable and, therefore, preventable. Combinations of interacting drugs are sometimes used intentionally with favorable effect—for example in patients with hypertension or neoplastic diseases. 

About the Authors

Drs. Philip Hansten and John Horn are recognized international authorities in the field of drug-drug interactions and have more than 75 years of combined experience in the field. The drug interactions books by Dr. Hansten and Dr. Horn have been translated into 6 languages and have sold more than one million copies worldwide since 1971. The authors have lectured extensively in North America, as well as in more than two-dozen countries throughout the world.

The authors and publisher of this website have made every effort to ensure the accuracy of the information presented.  Nevertheless, it is the responsibility of the reader to assess the appropriateness of the recommendations presented after consideration of patient specific factors and after consideration of any new developments in the field.  The authors and publisher disclaim all responsibility for any errors or damage incurred as a consequence of the use of the information contained in this website.

Copyright 2021 by H&H Publications.  All rights reserved.