Verapamil

Verapamil is a synthetic small-molecule drug belonging to the calcium channel blocker family; within that family it is a non-dihydropyridine of the phenylalkylamine subclass and functions as a class IV antiarrhythmic agent ^[1]. Unlike the dihydropyridine calcium blockers, which mainly relax blood vessels, verapamil also has strong effects on the electrical tissues of the heart ^[1]. It is taken by mouth as immediate-release and long-acting formulations and can also be given by injection.

Its established medical uses include lowering high blood pressure, relieving the chest pain of angina, and controlling or terminating supraventricular tachycardias by slowing conduction through the atrioventricular node ^[1]. It is regarded as the preventive drug of choice for cluster headache, a severe headache disorder, although the doses used for this purpose are often higher than those used in heart disease and call for monitoring of the heart's electrical intervals ^[2]. Verapamil has also been examined in other settings; a meta-analysis in recent-onset type 1 diabetes found that it helped preserve the body's own insulin-producing capacity over the first year, as measured by C-peptide levels ^[3].

Verapamil was introduced in the early 1960s and gained prominence over the following decades as one of the first agents shown to selectively interfere with the transport of calcium across cell membranes ^[1]. It received regulatory approval long ago and is now available as a generic medicine, and it appears on the World Health Organization's List of Essential Medicines. Both immediate-release and extended-release oral products exist, alongside an injectable form for hospital use.

Common side effects include constipation, dizziness, nausea, low blood pressure, headache and swelling of the ankles ^[1]. Because it slows the heart and reduces its force of contraction, it can worsen certain conduction problems and heart failure, and it is generally avoided in people with severe left-ventricular dysfunction, very low blood pressure, or specific pre-excitation arrhythmias such as Wolff-Parkinson-White syndrome accompanied by atrial fibrillation ^[1]. Verapamil is extensively processed by the liver and can interact with a number of other medicines ^[1].

Verapamil is prescription only. Constipation is very common; other effects include low blood pressure, dizziness, flushing, headache, and ankle swelling. Because it slows the heart, it can cause bradycardia or heart block and is dangerous when stacked with beta blockers or in people with certain conduction problems. It is avoided in heart failure with reduced ejection fraction because it further weakens contraction. It raises blood levels of digoxin and interacts with many CYP3A4 drugs and grapefruit juice. Do not stop abruptly in angina patients. Overdose causes profound low blood pressure and bradycardia and is a medical emergency.

Verapamil is a CYP3A4 substrate and an inhibitor of both CYP3A4 and P-glycoprotein, so it interacts in both directions.

The cardiac interactions come first. With beta blockers it adds negative inotropic and dromotropic effects, and the combination can cause severe bradycardia, AV block, hypotension and heart failure; intravenous verapamil given after a beta blocker is particularly hazardous. Ivabradine and dantrolene are both contraindicated with it.

By inhibiting P-glycoprotein, verapamil raises digoxin concentrations substantially, and digoxin toxicity follows. Colchicine accumulates through the same transporter plus CYP3A4, and the result can be neuromyopathy or marrow suppression. Simvastatin, lovastatin and atorvastatin build up through CYP3A4 inhibition, raising myopathy and rhabdomyolysis risk.

In the other direction, rifampin, phenytoin, carbamazepine and St John's wort can strip verapamil of most of its effect, while grapefruit juice, clarithromycin and azole antifungals raise it. Verapamil also raises carbamazepine levels.

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Verapamil was first synthesized in 1962 by the German pharmaceutical company Knoll AG, where chemist Ferdinand Dengel prepared it as a derivative intended to relax coronary vessels. Its true mechanism became clear only after the physiologist Albrecht Fleckenstein, working in Germany during the mid-1960s, demonstrated that the compound blocked the movement of calcium into heart muscle; this insight established the very concept of calcium antagonism and made verapamil the prototype of the calcium channel blocker class.

It entered clinical use as an antianginal and antiarrhythmic agent and was later classified as a class IV antiarrhythmic and a phenylalkylamine, distinguishing it from the dihydropyridine blockers. Verapamil subsequently became a first-line preventive treatment for cluster headache, a role reaffirmed in modern European neurology guidelines. It appears on the World Health Organization's list of essential medicines and remains in broad use more than half a century after its introduction.

Verapamil is regarded as a foundational cardiovascular medicine, credited with opening the entire field of calcium channel pharmacology. It is trusted for slowing rapid heart rhythms that arise above the ventricles and for easing angina and high blood pressure, acting directly at the calcium channel rather than through the autonomic nervous system. Among neurologists it holds a distinctive place as the preferred prophylactic drug for cluster headache, one of the most painful conditions in medicine, where it can substantially reduce attack frequency. Its long track record gives clinicians confidence, although its tendency to slow cardiac conduction means it must be used carefully alongside beta-blockers and in patients with certain conduction abnormalities, and constipation is a common nuisance. Decades of accumulated experience keep it a dependable, versatile agent.