Saturday, September 15, 2018

Think before you prescribe - Knowledge of drug interactions may prevent undesirable clinical outcomes

Drug interactions occur when two drugs are taken at the same time and one drug affects the activity of the other drug resulting in an undesired clinical outcome in the form of either adverse effects or treatment failure or enhanced action of the other drug. Some drug interactions may even be potentially life-threatening. Drug interactions may also occur between drugs and food or between drugs and herbs.

Different drugs are absorbed differently. Some need to be taken without food i.e. on an empty stomach, while some drugs must be taken with food. Some drugs alter the absorption of the other drug by changing the pH.  For example, concomitant administration of itraconazole capsules and proton pump inhibitors such as omeprazole or H2receptor antagonists such as famotidine and ranitidine impairs absorption of itraconazole resulting in decreased levels (Clin Infect Dis. 2009;48(10):1441-58). Itraconazole capsules should be taken after a full meal for optimal absorption. The PPI or H2RA should be taken at least 1 hour before or 2 hours after itraconazole capsules. But, itraconazole solution should be taken on an empty stomach as it does not require acidic pH or food to increase its absorption.

Metoclopramide increases gut motility and may decrease digoxin absorption by accelerating gastric emptying (J Res Med Sci. 2013;18:601-10) necessitating an increase in digoxin dose. (US National Library of Medicine. Dailymed. Digoxin tablet).

Iron or calcium can bind to levothyroxine in the digestive tract when taken together and reduce its absorption and consequently its efficacy (Ann Intern Med. 1992;117:1010-3; JAMA. 2000;283:2822-5). Hence, they should be taken at least 4 hours apart from levothyroxine. Concurrent cow's milk ingestion also reduces absorption of oral levothyroxine (Thyroid. 2018 Apr;28(4):454-457). Patients are usually advised to levothyroxine in the morning 30-60 minutes before breakfast (Caspian J Intern Med. 2015;6(3):134-40).

Drug interactions are avoidable. Knowledge of drug-drug, drug-food and drug-herb interactions can avoid potential harm to the patient. Drug interactions should be kept in mind as differential diagnosis if an adverse effect occurs or the desired therapeutic response is not achieved.

Dr KK Aggarwal
Padma Shri Awardee
Vice President CMAAO
Group Editor-in-Chief IJCP Publications
President Heart Care Foundation of India
Immediate Past National President IMA

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