American Board of Psychiatry and Neurology (ABPN) Practice Exam 2026 - Free ABPN Practice Questions and Study Guide

Erythromycin is known to significantly increase the levels of carbamazepine when the two medications are co-administered. This is primarily due to erythromycin's effect on the cytochrome P450 enzyme system, specifically the CYP3A4 isoenzyme, which plays a crucial role in the metabolism of carbamazepine. When erythromycin inhibits this enzyme, it leads to decreased metabolism and clearance of carbamazepine, resulting in higher plasma concentrations of the drug.

Understanding this interaction is important because elevated levels of carbamazepine can lead to toxicity, with symptoms such as dizziness, nausea, vomiting, and even more severe neurological effects. Therefore, careful monitoring and potential dosage adjustments of carbamazepine are warranted when starting or continuing erythromycin therapy.

Conversely, the other options listed do not have the same significant effect on carbamazepine levels. For instance, cyclosporine and phenobarbital primarily affect different metabolic pathways and do not cause a remarkable increase in carbamazepine levels. Doxycycline, an antibiotic as well, does not have a significant impact on carbamazepine metabolism, underscoring the unique interaction between erythromycin and carbamazepine.