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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