WASHINGTON, Sept 23 (Reuters) - An epilepsy drug may help many patients with migraine by preventing nerves on the brain from becoming over-excited, a headache specialist said on Monday.

The drug, called topiramate, may help patients who do not respond to more traditional migraine drugs, said Dr. Stephen Silberstein, director of the Jefferson Headache Center at Thomas Jefferson University in Philadelphia.

"Migraine is due to a more sensitive brain," said Silberstein, who was scheduled to present his findings to a meeting in Britain on Monday of the Migraine Trust.

Several migraine experts have described a process known as sensitization, during which pain starts in the nerves on the outside of the brain. If the pain is not stopped right away, the effect moves to nerves that normally do not perceive pain.

Non-painful stimuli are perceived as painful by the patient, who may say things like "my hair hurts". This strange sensation is called cutaneous allodynia.

Triptan drugs used to treat migraines can prevent this from happening, but cannot reverse it once it starts. Beta-blockers, narcotics and ergot derivatives, each with a different mechanism of action, are also used to treat migraine.

"Different types of migraine respond to different types of medication," Silberstein told a seminar organized by the American Medical Association.

Migraine patients often know they can stop a migraine if they take drugs right away, but the specialized drugs are expensive and can have severe side-effects. Sometimes a simple aspirin will work in very early migraine in some patients, but there is the risk of overuse.

People who take analgesics such as aspirin, ibuprofen or other drugs can develop chronic daily headaches caused by the drugs themselves.

"If something hurts, you don’t want to wait until it’s so bad to treat it," Silberstein said. Patients often walk a fine line and may wait until it is too late to take a painkiller.

Silberstein hoped topiramate might help prevent the sensitization process from starting in the first place. He tested around 500 migraine patients, who got either a placebo or topiramate in one of three different doses.

Of the patients who took dummy pills, 23 percent said their migraines got better. Thirty-six percent on the lowest dose of topiramate, 50 mg, reported an improvement, as did just over half of those who got 100 mg or 200 mg of the drug.

He noticed some of the same memory side-effects that topiramate has had in other trials. These include "language problems (primarily articulation or ‘word-finding’)," said the makers of the drug, Ortho-McNeil Pharmaceutical Inc., a subsidiary of Johnson & Johnson. Topiramate is sold under the name Topamax by Ortho-McNeil. Johnson & Johnson helped sponsor the AMA seminar where Silberstein spoke.

"What was amazing in this trial was the effect on weight," said Silberstein. The patients who got topiramine lost, on average, 3.8 percent of their body weight. In February Ortho-McNeil said it was halting clinical trials testing Topamax to treat obesity while it reformulated the drug because of its side-effects, including memory problems, fatigue, sleepiness, difficulty in concentration and tingling in the fingers and toes.

Earlier this month it stopped a test of the drug against bipolar disorder because it did not work well enough.