Penicillin: Erythromycin, in low bacteriostatic concentrations, may inhibit the actions of bactericidal drugs. In high concentrations, Erythromycin may act synergistically with penicillin.
Use of Erythromycin in patients receiving digoxin, warfarin, carbamazepine or high doses of theophylline may result in potentiation of the effects due to impairment of excretion. A possible interaction between Erythromycin and Vinblastine has been reported in which patients receiving the two agents concurrently may experience myalgia, neutropenia and fever. It is recommended that patients should avoid receiving Erythromycin and vinblastine at the same time.
When administered concurrently with Erythromycin, increases in serum concentrations may occur for the drugs metabolised by the cytochrome P450 system such as alfentanil, bromocriptine, hexobarbitone, midazolam, phenytoin, tacrolimus, triazolam, valproate, acenocoumarol. Appropriate monitoring should be undertaken and dosage should be adjusted as necessary. Concomitant use of simvastatin or lovastatin with Erythromycin is contraindicated (see Contraindications and Precautions) and Erythromycin can lead to increased serum level of other statins by inhibiting the CYP3A4 isoenzyme.
Increased plasma levels of cyclosporin may occur in patients on Erythromycin.
Ergotism has been reported in patients receiving Erythromycin in combination with ergot.
Concomitant use of Erythromycin with astemizole, terfenadine, cisapride or pimozide is likely to result in an increased risk of cardiotoxicity with these drugs, and is therefore contraindicated (see Contraindications).
Concomitant use of theophylline and Erythromycin has resulted in increased theophylline levels resulting in toxicity and decreased Erythromycin levels which could lead to subtherapeutic concentrations.
Concomitant use of Erythromycin and medications that cause prolongation of the QT interval has been reported to lead to cardiac toxicity or Torsades de Pointes.
Caution should be used with the concomitant use of Erythromycin with statins.
Interactions between Erythromycin and verapamil or clozapine have been reported.
Laboratory tests used for measurement of urinary catecholamines, SGOT and 17-hydroxycorticosteroids, may be affected if a colorimetric test is used. This interference may complicate interpretation of liver function tests. Suppression of growth of Lactobacillus casei by Erythromycin, may interfere with serum folate measurements.