indication

For the treatment of hypertension, angina, and cluster headache prophylaxis.

pharmacology

Verapamil is an L-type calcium channel blocker that also has antiarrythmic activity. The R-enantiomer is more effective at reducing blood pressure compared to the S-enantiomer. However, the S-enantiomer is 20 times more potent than the R-enantiomer at prolonging the PR interval in treating arrhythmias.

mechanism of action

Verapamil inhibits voltage-dependent calcium channels. Specifically, its effect on L-type calcium channels in the heart causes a reduction in ionotropy and chronotropy, thuis reducing heart rate and blood pressure. Verapamil's mechanism of effect in cluster headache is thought to be linked to its calcium-channel blocker effect, but which channel subtypes are involved is presently not known.

toxicity

LD₅₀=8 mg/kg (i.v. in mice)

absorption

90%

half life

2.8-7.4 hours

route of elimination

Approximately 70% of an administered dose is excreted as metabolites in the urine and 16% or more in the feces within 5 days. About 3% to 4% is excreted in the urine as unchanged drug.

drug interactions

Acebutolol: Increased effect of both drugs

Amifostine: Verapamil may enhance the hypotensive effect of Amifostine. At chemotherapeutic doses of Amifostine, Verapamil should be withheld for 24 hours prior to Amifostine administration. Caution should be used at lower Amifostine doses used during radiotherapy, but routine interruption of Verapamil therapy is not recommended.

Aminophylline: Verapamil increases the effect of theophylline

Amiodarone: Additive bradycardic effects may occur. One case report of sinus arrest has been reported. Monitor for changes in the therapeutic effect and signs of Verapamil toxicity if Amiodarone is initiated, discontinued or dose changed.

Amobarbital: Amobarbital, a CYP3A4 inducer, may increase the serum concentration of Verapamil, a CYP3A4 substrate. Monitor for changes in the therapeutic/adverse effects of Verapamil if Amobarbital is initiated, discontinued or dose changed.

Amprenavir: Amprenavir, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Amprenavir is initiated, discontinued or dose changed.

Atazanavir: Atazanavir, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Atazanavir is initiated, discontinued or dose changed.

Atenolol: Increased effect of both drugs

Atorvastatin: Verapamil, a moderate CYP3A4 inhibitor, may increase the serum concentration of Atorvastatin by decreasing its metabolism. Avoid concurrent use if possible or reduce lovastatin dose during concomitant therapy. Monitor for changes in the therapeutic/adverse effects of Atorvastatin if Verapamil is initiated, discontinued or dose changed.

Bisoprolol: Increased effect of both drugs

Bromazepam: Verapamil may increase the serum concentration of bromazepam by decreasing its metabolism. Consider alternate therapy or a reductin in the bromazepam dose. Monitor for changes in the therapeutic and adverse effects of bromazepam if verapamil is initiated, discontinued or dose changed.

Buspirone: Verapamil may increase the serum concentration of Buspirone. The likely occurs via Verapamil-mediated CYP3A4 inhibition resulting in decreased Buspirone metabolism. Monitor for changes in the therapeutic/adverse effects of Buspirone if Verpamil is initiated, discontinued or dose changed.

Butabarbital: Butabarbital, a CYP3A4 inducer, may increase the serum concentration of Verapamil, a CYP3A4 substrate. Monitor for changes in the therapeutic/adverse effects of Verapamil if Butabarbital is initiated, discontinued or dose changed.

Butalbital: Butalbital, a CYP3A4 inducer, may increase the serum concentration of Verapamil, a CYP3A4 substrate. Monitor for changes in the therapeutic/adverse effects of Verapamil if Butalbital is initiated, discontinued or dose changed.

Carbamazepine: Verapamil may increase the serum concentration of Carbamazepine by decreasing its metabolism. Monitor for changes in the therapeutic/adverse effects of Carbamazepine if Verapamil is initiated, discontinued or dose changed.

Carvedilol: Increased effect of both drugs

Clarithromycin: Clarithromycin, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Clarithromycin is initiated, discontinued or dose changed.

Colchicine: Verapamil may increase the serum concentration of Colchicine. This likely occurs via Verapamil-mediated inhibition of CYP3A4 and p-glycoprotein-mediated transport. Monitor for changes in the therapeutic/adverse effects of Colchicine if Verapamil is initiated, discontinued or dose changed.

Conivaptan: Conivaptan, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Conivaptan is initiated, discontinued or dose changed.

Cyclosporine: Verapamil may increase the serum concentration of cyclosporine by inhibiting CYP3A4-mediated metabolism of cyclosporine. Monitor for changes in the therapeutic/adverse effects of cyclosporine if verapamil is initiated, discontinued or dose changed.

Dabigatran etexilate: Verapamil may increase serum concentrations of the active metabolite(s) of dabigatran etexilate, resulting in an increased risk of bleeding. Therapy modification should be considered.

Darunavir: Darunavir, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Darunavir is initiated, discontinued or dose changed.

Delavirdine: Delavirdine, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Delavirdine is initiated, discontinued or dose changed.

Digitoxin: Verapamil may increase the serum concentration of Digitoxin by decreasing its metabolism and clearance. Monitor for changes in the therapeutic/adverse effects of Digitoxin if Verpamail is initiated, discontinued or dose changed.

Digoxin: Verapamil may increase the serum concentration of Digoxin by decreasing its metabolism and clearance. Monitor for changes in the therapeutic/adverse effects of Digoxin if Verpamail is initiated, discontinued or dose changed.

Dofetilide: Verapamil may increase the plamsa levels of Dofetilide. Increased risk of torsade de pointes. Concomitant therapy is contraindicated.

Eplerenone: This CYP3A4 inhibitor increases the effect and toxicity of eplerenone

Erythromycin: Erythromycin, a moderate CYP3A4 inhibitor, may increase the serum concentration of veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Monitor for changes in the therapeutic/adverse effects of verapamil if erythromycin is initiated, discontinued or dose changed.

Esmolol: Increased effect of both drugs

Everolimus: Concomitant administration may increase the serum concentrations of both agents. Concurrent use should be avoided.

Fluconazole: Fluconazole may increase the serum concentration of Verapamil by decreasing Verapamil metabolism. This likely occurs via Fluconazole-mediated CYP3A4 inhibition. Monitor for changes in the therapeutic/adverse effects of Verapamil if Fluconazole is initiated, discontinued, or dose changed.

Fosamprenavir: Fosamprenavir, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Fosamprenavir is initiated, discontinued or dose changed.

Halofantrine: Verapamil, a moderate CYP3A4 inhibitor, may increase the serum concentration of Halofantrine by decreasing its metabolism. Extreme caution with increased cardiac status monitoring should be used during concomitant therapy.

Imatinib: Imatinib, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Imatinib is initiated, discontinued or dose changed.

Indinavir: Indinavir, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Indinavir is initiated, discontinued or dose changed.

Isoniazid: Isoniazid, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Isoniazid is initiated, discontinued or dose changed.

Itraconazole: Itraconazole, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Itraconazole is initiated, discontinued or dose changed.

Ketoconazole: Ketoconazole, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Ketoconazole is initiated, discontinued or dose changed.

Labetalol: Increased effect of both drugs

Lithium: Signs of lithium toxicity

Lopinavir: Lopinavir, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Lopinavir is initiated, discontinued or dose changed.

Lovastatin: Verapamil, a moderate CYP3A4 inhibitor, may increase the serum concentration of Lovastatin by decreasing its metabolism. Avoid concurrent use if possible or reduce lovastatin dose during concomitant therapy. Monitor for changes in the therapeutic/adverse effects of Lovastatin if Verapamil is initiated, discontinued or dose changed.

Methohexital: Methohexital, a CYP3A4 inducer, may increase the serum concentration of Verapamil, a CYP3A4 substrate. Monitor for changes in the therapeutic/adverse effects of Verapamil if Methohexital is initiated, discontinued or dose changed.

Methylphenobarbital: Methylphenobarbital, a CYP3A4 inducer, may increase the serum concentration of Verapamil, a CYP3A4 substrate. Monitor for changes in the therapeutic/adverse effects of Verapamil if Methylphenobarbital is initiated, discontinued or dose changed.

Metoprolol: Increased effect of both drugs

Miconazole: Miconazole, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Miconazole is initiated, discontinued or dose changed.

Midazolam: Verapamil may increase the serum concentration of Midazolam by decreasing its metabolism. Avoid concomitant therapy if possible or consider a dose reduction in the initial dose of Midazolam.

Nadolol: Increased effect of both drugs

Nafcillin: Nafcillin may decrease the serum concentration of Verapamil by increasing its metabolism via CYP3A4. Monitor for changes in the therapeutic/adverse effects of Verapamil if Nafcillin is initiated, discontinued or dose changed.

Nefazodone: Nefazodone, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Nefazodone is initiated, discontinued or dose changed.

Nelfinavir: Nelfinavir, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Nelfinavir is initiated, discontinued or dose changed.

Nicardipine: Nicardipine, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Nicardipine is initiated, discontinued or dose changed.

Oxprenolol: Increased effect of both drugs

Oxtriphylline: Verapamil increases the effect of theophylline

Pentobarbital: Pentobarbital, a CYP3A4 inducer, may increase the serum concentration of Verapamil, a CYP3A4 substrate. Monitor for changes in the therapeutic/adverse effects of Verapamil if Pentobarbital is initiated, discontinued or dose changed.

Phenobarbital: Phenobarbital, a CYP3A4 inducer, may increase the serum concentration of Verapamil, a CYP3A4 substrate. Monitor for changes in the therapeutic/adverse effects of Verapamil if Phenobarbital is initiated, discontinued or dose changed.

Phenytoin: Verapamil may increase the serum concentration of Phenytoin by decreasing its metabolism. Monitor for changes in the therapeutic/adverse effects of Phenytoin if Verapamil is initiated, discontinued or dose changed.

Pindolol: Increased effect of both drugs

Posaconazole: Posaconazole, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Posaconazole is initiated, discontinued or dose changed.

Prazosin: Risk of hypotension at the beginning of therapy

Primidone: The barbiturate, primidone, decreases the effect of the calcium channel blocker, verapamil.

Propranolol: Increased effect of both drugs

Quinidine: Concurrent therapy may result in increased serum levels of both agents. Both agents are CYP3A4 inhibitors and substrates. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of the agent if the other is initiated, discontinued or dose changed.

Quinupristin: This combination presents an increased risk of toxicity

Ranolazine: Verapamil, a CYP3A4 inhibitor, may increase the serum concentration of Ranolazine. Concomitant therapy is contraindicated.

Rifabutin: Rifabutin, a CYP3A4 inducer, may decrease the serum concentration of Verapamil by increasing its metabolism (particularly in the intestinal mucosa) and decreasing its absorption. Monitor for changes in the therapeutic/adverse effects of Verapamil if Rifabutin is initiated, discontinued or dose changed.

Rifampin: Rifampin, a CYP3A4 inducer, may decrease the serum concentration of Verapamil by increasing its metabolism (particularly in the intestinal mucosa) and decreasing its absorption. Monitor for changes in the therapeutic/adverse effects of Verapamil if Rifampin is initiated, discontinued or dose changed.

Rifapentine: Rifapentine, a CYP3A4 inducer, may decrease the serum concentration of Verapamil by increasing its metabolism (particularly in the intestinal mucosa) and decreasing its absorption. Monitor for changes in the therapeutic/adverse effects of Verapamil if Rifapentine is initiated, discontinued or dose changed.

Ritonavir: Ritonavir, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Ritonavir is initiated, discontinued or dose changed.

Rituximab: Verapamil may increase the hypotensive effects of Rituximab. Consider withholding Verapamil therapy for 12 hours prior to Rituximab infusion.

Saquinavir: Saquinavir, a strong CYP3A4 inhibitor, may increase the serum concentration of Veramapil, a CYP3A4 substrate, by decreasing its metabolism and clearance. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Verapamil if Saquinavir is initiated, discontinued or dose changed.

Secobarbital: Secobarbital, a CYP3A4 inducer, may increase the serum concentration of Verapamil, a CYP3A4 substrate. Monitor for changes in the therapeutic/adverse effects of Verapamil if Secobarbital is initiated, discontinued or dose changed.

Simvastatin: Verapamil, a moderate CYP3A4 inhibitor, may increase the serum concentration of Simvastatin by decreasing its metabolism. Avoid concurrent use if possible or reduce Simvastatin dose during concomitant therapy. Monitor for changes in the therapeutic/adverse effects of Simvastatin if Verapamil is initiated, discontinued or dose changed.

Tacrolimus: The calcium channel blocker, Verapamil, may increase the blood concentration of Tacrolimus. Monitor for changes in the therapeutic/toxic effects of Tacrolimus if Verapamil therapy is initiated, discontinued or altered.

Tamsulosin: Verapamil, a CYP3A4 inhibitor, may decrease the metabolism and clearance of Tamsulosin, a CYP3A4 substrate. Monitor for changes in therapeutic/adverse effects of Tamsulosin if Verapamil is initiated, discontinued, or dose changed.

Telithromycin: Telithromycin, a CYP3A4 and p-glycoprotein inhibitor, may increase the Vinblastine serum concentration and distribution in certain cells. Consider alternate therapy to avoid Vinblastine toxicity. Monitor for changes in the therapeutic/adverse effects of Vinblastine if Telithromycin is initiated, discontinued or dose changed.

Terfenadine: Increased risk of cardiotoxicity and arrhythmias

Theophylline: Verapamil increases the effect of theophylline

Thiopental: Thiopental, a CYP3A4 inducer, may increase the serum concentration of Verapamil, a CYP3A4 substrate. Monitor for changes in the therapeutic/adverse effects of Verapamil if Thiopental is initiated, discontinued or dose changed.

Timolol: Additive effects of decreased heart rate and contractility may occur. Increased risk of heart block.

Tipranavir: Tipranavir, co-administered with Ritonavir, may alter the concentration of Verapamil. Monitor for efficacy and adverse/toxic effects of Verapamil.

Tolterodine: Verapamil may decrease the metabolism and clearance of Tolterodine. Adjust Tolterodine dose and monitor for efficacy and toxicity.

Topotecan: The p-glycoprotein inhibitor, Verapamil, may increase the bioavailability of oral Topotecan. A clinically significant effect is also expected with IV Topotecan. Concomitant therapy should be avoided.

Tramadol: Verapamil may increase Tramadol toxicity by decreasing Tramadol metabolism and clearance.

Trazodone: The CYP3A4 inhibitor, Verapamil, may increase Trazodone efficacy/toxicity by decreasing Trazodone metabolism and clearance. Monitor for changes in Trazodone efficacy/toxicity if Verapamil is initiated, discontinued or dose changed.

Treprostinil: Additive hypotensive effect. Monitor antihypertensive therapy during concomitant use.

Triazolam: Verapamil may increase the serum concentration of Triazolam by decreasing its metabolism. Avoid concomitant therapy if possible or consider a dose reduction in the initial dose of Triazolam.

Voriconazole: Voriconazole, a strong CYP3A4 inhibitor, may increase the serum concentration of verapamil by decreasing its metabolism. Monitor for changes in the therapeutic and adverse effects of verapamil if voriconazole is initiated, discontinued or dose changed.