WASHINGTON (Reuters) - The U.S. Food and Drug Administration has issued a warning to patients and physicians about the increased risk of seizures in multiple sclerosis patients taking the drug Ampyra.

The drug was developed by Acorda Therapeutics Inc to improve walking ability in patients with multiple sclerosis. Biogen Idec Inc has rights to the drug in Europe where it is sold under the brand name Fampyra.

The FDA said that based on reports, it recently evaluated seizure risk in MS patients taking Ampyra and noted that the majority of seizures happened within days to weeks after starting the recommended dose and occurred in patients with no history of seizures.

Seizures are a known risk with Ampyra, the agency said, and the risk increases with higher blood levels of the drug. Since Ampyra is eliminated from the kidneys, patients with kidney impairment may develop higher blood levels of the drug, thus increasing their seizure risk, the FDA said.

As a result, the agency is updating the prescribing information for physicians, making clear that a patient's kidney function should be checked before starting Ampyra. Patients should be monitored at least once a year while the treatment continues.

In addition, patients who miss a dose should not take extra doses as an extra dose can increase seizure risk, the agency said.

The drug was approved in the United States in January 2010. Biogen received conditional approval for drug in Europe last July. Conditional approval is granted to drugs in which the benefit is seen to outweigh the risk but where additional information is needed to confirm that view.

As part of the conditions of approval, regulators recommended Biogen carry out an additional study to discover more about the drug's benefits and safety over the long term.