Brand-Specific Medication Intolerance: A Case Series Highlighting Excipient Allergies in Clinical Practice

Drug allergies are commonly attributed to active pharmaceutical ingredients, but excipients; the inactive ingredients comprising up to 90% of medication formulations, can also cause adverse reactions that are often overlooked. The recent cough syrup tragedy in Oct 2025 in India, due to an excipient, underlines the seriousness of the matter. We present five patients who experienced allergic reactions to specific medication brands but tolerated identical active ingredients from different manufacturers. All patients showed complete resolution when switched to alternative brands containing different excipients. Cases included reactions to phenobarbital, diclofenac, aspirin, and rabeprazole formulations. Analysis of 200 tablet formulations revealed magnesium stearate, lactose, and microcrystalline cellulose as the most common excipients. Excipient allergies represent an under recognized cause of drug reactions. Clinicians should consider excipient sensitivity when patients report brand-specific medication intolerance, as this can prevent unnecessary active ingredient avoidance. Keywords: Drug Allergy, Excipients, Pharmaceutical Formulation, Adverse Drug Reactions, Case Series, Brand-Specific Intolerance