Increased risk of ventricular arrhythmias/rhythm disorders, particularly Torsades de pointes w/ class Ia (quinidine, hydroquinidine, disopyramide) & III (dofetilide, ibutilide, sotalol) antiarrhythmics; arsenic compd, bepridil, cisapride, citalopram, escitalopram, diphemanil, dolasetron IV, domperidone, dronedarone, erythromycin IV, levofloxacin, mequitazine, mizolastine, moxifloxacin, prucalopride, spiramycin IV, toremifene, vincamine IV; antiparasitics (halofantrine, lumefantrine, pentamidine); neuroleptics (amisulpride, chlorpromazine, cyamemazine, droperidol, flupentixol, fluphenazine, haloperidol, levomepromazine, pimozide, pipamperone, pipotiazine, sertindole, sulpiride, sultopride, tiapride, zuclopenthixol); methadone; fluoroquinolones; stimulant laxatives; β-blockers in heart failure (bisoprolol, carvedilol, metoprolol, nebivolol); macrolides (azithromycin, clarithromycin, roxithromycin); hypokalemic diuretics (alone or in combination), amphotericin B (IV route), glucocorticoids (systemic route), tetracosactide; bradycardic agents; as metabolism may be decreased w/ voriconazole. Cardiac automaticity & conduction disorders w/ risk of excessive bradycardia w/ telaprevir; β-blockers in heart failure (bisoprolol, carvedilol, metoprolol, nebivolol); other β-blockers. Risk of increased adverse effects due to decreased metabolism w/ cobicistat. Possibly symptomatic or even fatal bradycardia w/ dual therapy w/ daclatasvir/sofosbuvir or ledipasvir/sofosbuvir. Increased plasma conc of CYP3A4 substrates, potentially increasing the toxicity of these substrates (lidocaine, tacrolimus, sildenafil, midazolam, dihydroergotamine, ergotamine, colchicine, triazolam); fidaxomicine; CYP2C9 substrates eg, vit K antagonists & phenytoin; flecainide by inhibiting cytochrome CYP2D6. Increased blood ciclosporin conc, due to reduced liver metabolism, w/ a risk of nephrotoxic effects. Risk of bradycardia & AV heart block w/ diltiazem inj & verapamil inj; particularly in the elderly w/ oral diltiazem & oral verapamil. Potentiation of bradycardia-inducing effects w/ potentially fatal outcome w/ fingolimod. Inhibits P-glycoprotein & may lead to increased exposure of P-glycoprotein substrates. Suppressed automaticity (excessive bradycardia) & AV conduction disorders w/ digitalis drugs (digoxin). Increased plasma dabigatran conc, w/ a higher risk of bleeding. Increased effect & risk of bleeding of vit K antagonist. Increased plasma phenytoin conc w/ signs of overdose, particularly neurological signs (decreased liver metabolism of phenytoin). Increased risk of muscle toxicity (eg, rhabdomyolysis) w/ statins (simvastatin, atorvastatin, lovastatin). Risk of increased lidocaine plasma conc, potentially leading to neurological & cardiac adverse effects, due to decreased liver metabolism. Increased blood tacrolimus conc due to inhibition of its metabolism. Contractility, automaticity & conduction disorders (suppressed compensatory sympathetic mechanisms) w/ esmolol. Risk of decreased plasma conc of amiodarone & its active metabolite w/ orlistat. Risk of increased tamsulosin-induced adverse effects due to inhibition of its hepatic metabolism. Risk of excessive bradycardia (cumulative bradycardia-inducing effects) w/ pilocarpine.