drug interactions

Introduction

• Instructor: Gretchen K. Garofoli, PharmD, BCACP, CTTS, FAPhA

Learning Objectives

• Define the term "drug interaction"

• Identify foods that commonly interact with medications

• Categorize different types of drug interactions

• Determine the significance of drug interactions

• Utilize drug interaction identification software

Definition of Drug Interaction

• Drug Interaction: A pharmacologic or clinical response to a combination of drugs that differs from the expected effects of the individual drugs when taken alone. (Source: Tatro DS, Drug Interaction Facts, 1992)

Types of Interactions

Pharmacokinetic

• What the body does with the drug:

  • Absorption: How the drug is taken up by the body

  • Distribution: How the drug spreads in the body

  • Metabolism: How the drug is broken down by the body

  • Excretion: How the drug is eliminated from the body

• Common interactions involve Cytochrome P450 (CYP 450) enzymes.

Pharmacodynamic

• What the drug does to the body:

  • Additive, synergistic, or antagonist effects between drugs.

Categories of Substances Impacting Drug Interactions

• Prescription medications

• Over-the-counter medications

• Herbals and supplements

• Substances: Cigarettes, alcohol

• Foods: E.g., grapefruit juice, spinach

• Medical Conditions

Risk Factors for Drug Interactions

• Patients on multiple medications

• Patients consulting multiple physicians and using multiple pharmacies

• Usage of narrow therapeutic index medications

• Age: Pediatric and elderly patients

• Decreased renal or liver function

• Alcohol consumption

• Tobacco usage

• Specific dietary habits

• Use of herbal supplements

Severity of Drug Interactions

• Severe

• Major

• Moderate

• Minor

Common Drug-Drug Interactions

• MAOIs with other serotonergic drugs

• Potassium-sparing diuretics with ACE-Is/ARBs

• Bactrim® with warfarin

• Clarithromycin with digoxin or some statins

• Oral contraceptives with enzyme inducers

• Interactions involving narrow therapeutic index medications

• Calcium Carbonate and Proton Pump Inhibitors

• Iron and Levothyroxine

Common Drug-Disease Interactions

• Inappropriate medications in the Elderly

• Beer’s List medications

• Beta-Blockers in asthmatic patients

• Pioglitazone in patients with heart failure

• Nasal decongestants in patients with high blood pressure

• Metformin causing Vitamin B12 Deficiency

Common Drug-Food Interactions

• Tetracycline with milk

• Iron and levothyroxine with milk/calcium

• Warfarin with Vitamin K foods (spinach, kale, etc.)

• Statins with grapefruit juice

• Benzodiazepines, APAP, metronidazole with alcohol

• MAOIs with tyramine foods (chocolate, aged cheeses, sausage)

Drug Interaction Software

• Available platforms include:

  • Lexicomp®

  • MICROMEDEX®

  • Clinical Pharmacology®

  • Epocrates®

Handling Drug Interaction Alerts

• Monitor for medications with lingering effects, such as:

  • Amiodarone, fluoxetine, MAOIs

• Exercise caution with CYP450 Inhibitors/Inducers discontinuation

• Understand that class alerts may not apply to all medications within the class

Example Patient Medication List

• Duloxetine 60 mg

• Isosorbide MN ER 30 mg

• Januvia 50 mg

• Omeprazole 20 mg

• Aspirin 81 mg

• Clopidogrel 75 mg

• Atorvastatin 40 mg

Drug Interaction Case Studies

Testing Your Skills 1

• Scenario: Patient on clarithromycin and simvastatin

Testing Your Skills 2

• Scenario: Patient on tramadol and Zoloft

Testing Your Skills 3

• Scenario: Patient on amoxicillin and Ortho Tri-Cyclen

Common Issues in Drug Interaction Programs

• May fail to identify significant interactions

• May generate unnecessary alerts

The Pharmacist's Role

• Assess clinical significance of drug interactions

• Evaluate the necessity of overriding alerts

• Analyze drug interaction reports for relevance

• Counsel patients on potential interactions and outcomes

• Stay updated with literature for new interactions

Questions?