Erythromycin use in patients who are receiving high doses of
theophylline may be associated with an increase in serum theophylline levels and
potential theophylline toxicity. In case of theophylline toxicity and/or
elevated serum theophylline levels, the dose of theophylline should be reduced
while the patient is receiving concomitant erythromycin therapy.
Concomitant administration of erythromycin and digoxin has been reported to
result in elevated digoxin serum levels.
There have been reports of increased anticoagulant effects when erythromycin
and oral anticoagulants were used concomitantly. Increased anticoagulation
effects due to interactions of erythromycin with various oral anticoagulants may
be more pronounced in the elderly.
Erythromycin is a substrate and inhibitor of the 3A isoform subfamily of the
cytochrome P450 enzyme system (CYP3A). Coadministration of erythromycin and a
drug primarily metabolized by CYP3A may be associated with elevations in drug
concentrations that could increase or prolong both the therapeutic and adverse
effects of the concomitant drug. Dosage adjustments may be considered, and when
possible, serum concentrations of drugs primarily metabolized by CYP3A should be
monitored closely in patients concurrently receiving erythromycin.
The following are examples of some clinically significant CYP3A based drug
interactions. Interactions with other drugs metabolized by the CYP3A isoform are
also possible. The following CYP3A based drug interactions have been observed
with erythromycin products in post-marketing experience:
Ergotamine/dihydroergotamine
Concurrent use of erythromycin and ergotamine or
dihydroergotamine has been associated in some patients with acute ergot toxicity
characterized by severe peripheral vasospasm and dysesthesia.
Triazolobenzodiazepines (such as triazolam and alprazolam)
and related benzodiazepines
Erythromycin has been reported to decrease the clearance of
triazolam and midazolam, and thus, may increase the pharmacologic effect of
these benzodiazepines.
HMG-CoA Reductase Inhibitors
Erythromycin has been reported to increase concentrations of
HMG-CoA reductase inhibitors (e.g., lovastatin and simvastatin). Rare reports of
rhabdomyolysis have been reported in patients taking these drugs
concomitantly.
Sildenafil (Viagra)
Erythromycin has been reported to increase the systemic exposure
(AUC) of sildenafil. Reduction of sildenafil dosage should be considered. (See
Viagra package insert.)
There have been spontaneous or published reports of CYP3A based
interactions of erythromycin with cyclosporine, carbamazepine, tacrolimus,
alfentanil, disopyramide, rifabutin, quinidine, methylprednisolone, cilostazol,
vinblastine, and bromocriptine.
Concomitant administration of erythromycin with cisapride, pimozide,
astemizole, or terfenadine is contraindicated. (See CONTRAINDICATIONS.)
In addition, there have been reports of interactions of erythromycin with
drugs not thought to be metabolized by CYP3A, including hexobarbital, phenytoin,
and valproate.
Erythromycin has been reported to significantly alter the metabolism of the
nonsedating antihistamines terfenadine and astemizole when taken concomitantly.
Rare cases of serious cardiovascular adverse events, including
electrocardiographic QT/QTc interval prolongation,
cardiac arrest, torsades de pointes, and other ventricular arrhythmias have been
observed. (See CONTRAINDICATIONS.) In addition, deaths
have been reported rarely with concomitant administration of terfenadine and
erythromycin.
There have been post-marketing reports of drug interactions when erythromycin
was coadministered with cisapride, resulting in QT prolongation, cardiac
arrhythmias, ventricular tachycardia, ventricular fibrillation, and torsades de
pointes most likely due to the inhibition of hepatic metabolism of cisapride by
erythromycin. Fatalities have been reported. (See CONTRAINDICATIONS.)