Skip to Content

Drug Interactions

By

Shalini S. Lynch

, PharmD, University of California San Francisco School of Pharmacy

Last full review/revision Jul 2019| Content last modified Jul 2019

Drug interactions are changes in a drug’s effects due to recent or concurrent use of another drug or drugs (drug-drug interactions), ingestion of food (drug-nutrient interactions), or ingestion of dietary supplements (dietary supplement-drug interactions).

A drug-drug interaction may increase or decrease the effects of one or both drugs. Clinically significant interactions are often predictable and usually undesired (see Some Drugs With Potentially Serious Drug-Drug Interactions). Adverse effects or therapeutic failure may result. Rarely, clinicians can use predictable drug-drug interactions to produce a desired therapeutic effect. For example, coadministration of lopinavir and ritonavir to patients with HIV infection results in altered metabolism of lopinavir and increases serum lopinavir concentrations and effectiveness.

Some Drugs With Potentially Serious Drug-Drug Interactions*

Mechanism

Examples

Narrow margin of safety†

Antiarrhythmic drugs (eg, quinidine)

Antineoplastic drugs (eg, methotrexate)

Digoxin

Lithium

Theophylline

Warfarin

Extensive metabolism by certain hepatic enzymes

Alprazolam

Amitriptyline

Atorvastatin

Carbamazepine

Clozapine

Corticosteroids

Cyclosporine

Diazepam

HIV protease inhibitors

Imipramine

Lovastatin

Midazolam

Olanzapine

Phenytoin

Sildenafil

Simvastatin

Tacrolimus

Tadalafil

Theophylline

Triazolam

Vardenafil

Warfarin

Inhibition of certain hepatic enzymes‡

Aprepitant

Boceprevir

Cimetidine

Ciprofloxacin

Clarithromycin

Cobicistat

Conivaptan

Diltiazem

Erythromycin

Fluconazole

Fluoxetine

Fluvoxamine

Itraconazole

Ketoconazole

Paroxetine

Posaconazole

Ritonavir

Telaprevir

Telithromycin

Verapamil

Voriconazole

Induction of certain hepatic enzymes

Barbiturates (eg, phenobarbital)

Bosentan

Carbamazepine

Efavirenz

Phenytoin

Rifabutin

Rifampin

St. John’s wort

* Any drug to be used concurrently with one of these drugs should be thoroughly evaluated for possible interactions.

† Even when used alone, these drugs may have serious adverse effects. Concurrent use of another drug that increases the action of these drugs further increases risk of adverse effects. For additional research on potential drug-drug interactions, consult a reliable source, such as Drug.com's Drugs Interaction Checker.

‡ Inhibition also can occur after ingestion of grapefruit products.

In therapeutic duplication, 2 drugs with similar properties are taken at the same time and have additive effects. For example, taking a benzodiazepine for anxiety and another benzodiazepine at bedtime for insomnia may have a cumulative effect, leading to toxicity.

Drug interactions involve

In pharmacodynamic interactions, one drug alters the sensitivity or responsiveness of tissues to another drug by having the same (agonistic) or a blocking (antagonistic) effect. These effects usually occur at the receptor level but may occur intracellularly.

In pharmacokinetic interactions, a drug usually alters absorption, distribution, protein binding, metabolism, or excretion of another drug. Thus, the amount and persistence of available drug at receptor sites change. Pharmacokinetic interactions alter magnitude and duration, not type, of effect. They are often predicted based on knowledge of the individual drugs or detected by monitoring drug concentrations or clinical signs.

Minimizing drug interactions

Clinicians should know all of their patients’ current drugs, including drugs prescribed by other clinicians and all over-the-counter drugs, herbal products, and nutritional supplements. Asking patients relevant questions about diet and alcohol consumption is recommended. The fewest drugs in the lowest doses for the shortest possible time should be prescribed. The effects, desired and undesired, of all drugs taken should be determined because these effects usually include the spectrum of drug interactions. If possible, drugs with a wide safety margin should be used so that any unforeseen interactions do not cause toxicity.

Patients should be observed and monitored for adverse effects, particularly after a change in treatment; some interactions (eg, effects that are influenced by enzyme induction) may take ≥ 1 week to appear. Drug interactions should be considered as a possible cause of any unexpected problems. When unexpected clinical responses occur, prescribers should determine serum concentrations of selected drugs being taken, consult the literature or an expert in drug interactions, and adjust the dosage until the desired effect is produced. If dosage adjustment is ineffective, the drug should be replaced by one that does not interact with other drugs being taken.

Drugs Mentioned In This Article

Drug Name Select Trade
Clarithromycin BIAXIN
Amitriptyline No US brand name
Ciprofloxacin CILOXAN, CIPRO
Telithromycin KETEK
Carbamazepine TEGRETOL
methotrexate OTREXUP
Ketoconazole NIZORAL
Itraconazole SPORANOX
Erythromycin ERY-TAB, ERYTHROCIN
Theophylline ELIXOPHYLLIN
Posaconazole NOXAFIL
Atorvastatin LIPITOR
Voriconazole VFEND
Cyclosporine NEORAL, SANDIMMUNE
Fluconazole DIFLUCAN
Fluvoxamine LUVOX
Simvastatin ZOCOR
Paroxetine PAXIL
Tacrolimus PROGRAF
Aprepitant EMEND
Fluoxetine PROZAC, SARAFEM
Imipramine TOFRANIL
Vardenafil LEVITRA
Olanzapine ZYPREXA
Lovastatin ALTOPREV
Alprazolam XANAX
Cimetidine TAGAMET
Sildenafil VIAGRA
Boceprevir VICTRELIS
Conivaptan VAPRISOL
Verapamil CALAN
Tadalafil CIALIS
Triazolam HALCION
Phenytoin DILANTIN
ritonavir NORVIR
Diltiazem CARDIZEM, CARTIA XT, DILACOR XR
Rifabutin MYCOBUTIN
Clozapine CLOZARIL
Efavirenz SUSTIVA
Warfarin COUMADIN
Diazepam VALIUM
Bosentan TRACLEER
Rifampin RIFADIN, RIMACTANE
Digoxin LANOXIN
Lithium LITHOBID

Copyright © 2021 Merck & Co., Inc., known as MSD outside of the US, Kenilworth, New Jersey, USA. All rights reserved. Merck Manual Disclaimer