In vitro metabolic studies indicate that verapamil hydrochloride is metabolised by cytochrome P450 CYP3A4, CYP1A2, CYP2C8, CYP2C9 and CYP2C18. Verapamil has been shown to be an inhibitor of CYP3A4 enzymes and P-glycoprotein (P-gp). Clinically significant interactions have been reported with inhibitors of CYP3A4 (such as ketoconazole, erythromycin and ritonavir) causing elevation of plasma levels of verapamil hydrochloride while inducers of CYP3A4 have caused a lowering of plasma levels of verapamil hydrochloride, therefore, patients should be monitored for drug interactions.
Interactions between verapamil and the following medications have been reported:
Acetylsalicylic acid: Concomitant use of verapamil may increase the risk of bleeding.
Alcohol: Plasma concentration may be increased (see Effects on ability to drive and use machines)
Alpha blockers: Verapamil may increase the plasma concentrations of prazosin and terazosin which may have an additive hypotensive effect.
Antiarrhythmics: Verapamil may slightly decrease the plasma clearance of flecainide whereas flecainide has no effect on the verapamil plasma clearance. Verapamil may increase the plasma concentrations of quinidine. Pulmonary oedema may occur in patients with hypertrophic cardiomyopathy. The combination of verapamil and antiarrhythmic agents may lead to additive cardiovascular effects (e.g. AV block, bradycardia, hypotension, heart failure).
Anticoagulants: When oral verapamil was co-administered with dabigatran etexilate (150 mg), a P- gp substrate, the Cmax and AUC of dabigatran were increased but magnitude of this change differs depending on time between administration and the formulation of verapamil. Co- administration of verapamil 240 mg extended-release at the same time as dabigatran etexilate resulted in increased dabigatran exposure (increase of Cmax by about 90 % and AUC by about 70 %).
Close clinical surveillance is recommended when verapamil is combined with dabigatran etexilate and particularly in the occurrence of bleeding, notably in patients having a mild to moderate renal impairment.
Anticonvulsants: Verapamil may increase the plasma concentrations of carbamazepine. This may produce side effects such as diplopia, headache, ataxia or dizziness. Levels of verapamil may be reduced when taken with phenytoin.
Antidepressants: Verapamil may increase the plasma concentrations of imipramine.
Antidiabetics: Verapamil may increase the plasma concentrations of glibenclamide (glyburide).
Antihypertensives, diuretics, vasodilators: Potentiation of the hypotensive effect.
Anti-infectives: Rifampicin may reduce the plasma concentration of verapamil which may produce a reduced blood pressure lowering effect. Erythromycin, clarithromycin and telithromycin: May increase the plasma concentrations of verapamil.
Antineoplastics: Verapamil may increase the plasma concentrations of doxorubicin.
Barbiturates: Phenobarbital may reduce the plasma concentrations of verapamil.
Benzodiazepines and other anxiolytics: Verapamil may increase the plasma concentrations of buspirone and midazolam.
Beta blockers: Verapamil may increase the plasma concentrations of metoprolol and propranolol which may lead to additive cardiovascular effects (e.g. AV block, bradycardia, hypotension, heart failure). Intravenous beta-blockers should not be given to patients under treatment with verapamil.
Cardiac glycosides: Verapamil has been shown to increase the serum concentration of digoxin and digitoxin and caution should be exercised with regard to digitalis toxicity. The digitalis level should be determined and the glycoside dose reduced, if required.
Cimetidine: Increase in verapamil serum level is possible.
Colchicine: Colchicine is a substrate for both CYP3A and the efflux transporter, Pglycoprotein (P-gp). Verapamil is known to inhibit CYP3A and P-gp. When verapamil and colchicine are administered together, inhibition of P-gp and/or CYP3A by verapamil may lead to increased exposure to colchicine. Combined use is not recommended.
HIV antiviral agents: Due to the metabolic inhibitory potential of some of the HIV antiviral agents, such as ritonavir, plasma concentrations of verapamil may increase. Caution should be used or dose of verapamil may be decreased.
HMG Co-A Reductase Inhibitors (Statins): Verapamil may increase the plasma concentrations of simvastatin, atorvastatin and lovastatin. Treatment with HMG CoA reductase inhibitors (e.g. simvastatin, atorvastatin or lovastatin) in a patient taking verapamil should be started at the lowest possible dose and titrated upwards. If verapamil treatment is to be added to patients already taking an HMG CoA reductase inhibitor (e.g. simvastatin, atorvastatin or lovastatin), consider a reduction in the statin dose and re-titrate against serum cholesterol concentrations.
There is no direct in vivo clinical evidence of an interaction between atorvastatin and verapamil; however there is a strong potential for verapamil to significantly affect atorvastatin pharmacokinetics in a similar manner to simvastatin or lovastatin. Consider using caution when atorvastatin and verapamil are concomitantly administered.
Fluvastatin, pravastatin and rosuvastatin are not metabolised by CYP 3A4 and are less likely to interact with verapamil.
Immunosuppressants: Verapamil may increase the plasma concentrations of ciclosporin, everolimus, sirolimus and tacrolimus.
Inhaled anaesthetics: When used concomitantly, inhalation anaesthetics and calcium antagonists, such as verapamil hydrochloride, should each be titrated carefully to avoid additive cardiovascular effects (e.g. AV block, bradycardia, hypotension, heart failure).
Intravenous dantrolene: The association of this muscle relaxant given intravenously and verapamil is potentially dangerous (can cause fatal ventricular fibrillation in animals) and is contraindicated.
Lithium: Serum levels of lithium may be reduced (pharmacokinetic effect); there may be increased sensitivity to lithium causing enhanced neurotoxicity (pharmacodynamic effect).
Neuromuscular blocking agents employed in anaesthesia: The effects may be potentiated. The effects of verapamil may be additive to other hypotensive agents.
Serotonin receptor agonists: Verapamil may increase the plasma concentrations of almotriptan.
Theophylline: Verapamil may increase the plasma concentrations of theophylline.
Uricosurics: Sulfinpyrazone may reduce the plasma concentrations of verapamil which may produce a reduced blood pressure lowering effect.
Other Cardiac therapy: Concomitant use with ivabradine is contraindicated due to the additional heart rate lowering effect of verapamil to ivabradine (see section 4.3).
Other: St. John's Wort may reduce the plasma concentrations of verapamil, whereas grapefruit juice may increase the plasma concentrations of verapamil.
Co-administration of verapamil with metformin may reduce the efficacy of metformin.