The “party drug” ketamine could relieve the misery of millions of migraine sufferers, research suggests.

A US study found that taking the horse tranquilisers for up to a week reduced the severity of headaches for almost three in four sufferers.

The class B drugs, dubbed “Special K” by recreational users, may be an effective treatment for those who fail to respond to other medications, researchers said.

Migraine is the third most common condition in the world affecting around one in every five British women and around one in every 15 British men.

Dr Eric Schwenk, director of orthopaedic anesthesia at Thomas Jefferson University Hospital, Philadelphia, said: "Ketamine may hold promise as a treatment for migraine headaches in patients who have failed other treatments."

On average, pain scores - assessed on a scale of zero to 10 - more than halved - from 7.5 to 3.4 after patients were given the tranquiliser.

The findings presented at the Anesthesiology annual meeting in Boston came from a study of

61 male migraine sufferers in their 40s admitted to the hospital over a period of three years.

It found that almost three quarters experienced an improvement in their migraine intensity after inpatient treatment with ketamine, which involved regular infusions of the drug.