People with severe head injuries are usually put on the drug Dilantin for at least a year to prevent seizures but, according to a new study, the drug works only for the first week. After that it is useless and at any time it can cause side effects such as rashes, coordination disorders and mental slowing.

The findings, which emerged from the largest study ever of severe head injury patients, suggest that many of the 75,000 people hospitalized with such injuries in the United States each year should change their treatment regimen, researchers said.

"If I were a patient, I would see no reason to take {Dilantin, the brand name of phenytoin} after day seven" after such an injury, said Nancy Temkin, a researcher at the University of Washington Medical University of Washington Medical School researcher School in Seattle and the lead author of the report published in today's New England Journal of Medicine.

The study looked at 404 severe head-injury patients. Half were given phenytoin and half were given a placebo.

During the first week of the study, patients on the placebo were more than four times as likely to have a seizure than were patients on phenytoin.

But after the first week, there was no significant difference in seizure rates, the researchers said.

Between 5,000 and 30,000 of these patients with severe head injuries develop seizures, mostly within the first year.

The researchers said there is no other drug available as an alternative treatment.

"It's not like there's a whole lot of things that have been shown to be good," Temkin said.

Dilantin, or phenytoin, is also prescribed to prevent seizures from a variety of other causes but the study did not evaluate its effectiveness or safety in any of them.