A drug increasingly used to prevent cluster headaches can cause heart problems, according to a study in Neurology.

Those taking verapamil for cluster headaches should be closely monitored with frequent electrocardiograms to see if they've developed irregular heartbeats, the study authors conclude.

Cluster headaches are a severe though rare form of headache that is more common in men. The attacks usually occur in cyclical patterns, with frequent attacks over weeks or months generally followed by a period of remission when the headaches stop.

The study looked at 108 people (average age: 44) taking verapamil. They each had an ECG and then were prescribed an increase in the dosage of the drug every two weeks until the headaches were stopped or they started having side effects.

Twenty-one patients, or 19 per cent, developed problems with the electrical activity of the heart, or irregular heartbeats, while taking the drug.

Most of the cases were not considered serious; however, one person required a permanent pacemaker due to the problem.

A total of 37 per cent of the participants had slower than normal heart rates while on the drug, though the condition was severe enough in only four cases to warrant stopping the use of the drug.

"The benefit of taking verapamil to alleviate the devastating pain of cluster headaches has to be balanced against the risk of causing a heart abnormality that could progress into a more serious problem," said study author Dr. Peter Goadsby of the National Hospital for Neurology and Neurosurgery in London and the University of California, San Francisco.

Verapamil is used to treat angina (chest pain), high blood pressure, and irregular heartbeat (arrhythmia). It belongs to a group of drugs known as calcium channel blockers and while it is not clearly understood how it works, verapamil is thought to increase the blood supply to the heart.