MATTHEW PERRONE AP Business Writer WASHINGTON (AP) — Shares of Acorda Therapeutics plummeted Friday after the Food and Drug Administration issued a negative opinion that questioned the safety and effectiveness of the company's multiple sclerosis drug. A panel of FDA advisers is scheduled to meet next week to vote on Acorda's Fampridine-SR, which is designed to improve patients' ability to walk. Multiple sclerosis affects the brain and nervous system, causing loss of balance, muscle spasms and other movement problems. In briefing documents posted Friday, the FDA raised serious questions about whether the drug's benefits outweigh its risks, which include seizures. Acorda shares fell $4.76, or 21.4 percent, to close at $17.52 Friday. The company's stock has traded between $14.42 and $29.27 in the past year. Acorda conducted two trials tracking patients' ability to complete a 25-foot walk. While both studies met their goal of showing that patients on the drug had a statistical improvement in walking ability compared with those on placebo, the actual time to complete the exercise did not improve. "For these reasons, it appears the clinical meaning of the differences seen on the primary outcomes is in question," according to the FDA review. FDA scientists also noted an increased risk of seizures with the drug. The agency will ask its panel Wednesday whether the drug's benefits outweigh its side effects, including both seizures and urinary tract infections. The FDA is not required to follow the group's advice, though it usually does. Hawthorne, N.Y.-based Acorda stated in its own briefing documents that no evidence of seizures has been shown when the drug is given at the recommended 10-milligram dose. However, the company acknowledged that risks increase with higher doses. "Patients and physicians should therefore be thoroughly informed about the risks of prescribing or taking higher than the proposed 10 mg dose," the company said. Deutsche Bank analyst Mark Schoenbaum said most specialists who treat the disease "do not seem phased," by the drug's seizure risk. In a research note to investors, Schoenbaum predicted the FDA panel will vote to approve the drug, but with a "black box" warning about seizures. Acorda expects the FDA to complete its review of Fampridine-SR by Oct. 22.