The U.S. Food and Drug Administration said Thursday it was requiring manufacturers of metoclopramide, a drug used to treat gastrointestinal disorders, to add a boxed warning to their drug labels about the risk of developing a neurological disorder.

The agency said metoclopramide has been linked to tardive dyskinesia, a neurological disorder that can be a side effect of some types of drugs. The condition is marked by involuntary and repetitive movements of the body, even after the drugs are no longer taken. Current drug labels for metoclopramide already warn of the risk of developing tardive dyskinesia. But requirement of a boxed warning, which appears at the top of drug labels, means the agency is strengthening the warning.

The FDA said development of tardive dyskinsia "is directly related to the length of time a patient is taking metoclopramide and the number of doses taken." The FDA said the elderly and people who have been on the drug for a long time are at highest risk.

Metoclopramide, a generic drug, is used by more than two million Americans and is made by several drug makers including Baxter Healthcare Corp. (BAX), according to the FDA. The products are available in a variety of formulations including tablets, syrups and injections and are sold under brand names including Reglan Tablets, Reglan Oral Disintegrating Tablets, Metoclopramide Oral Solution and Reglan Injection.

"The chronic use of metoclopramide therapy should be avoided in all but rare cases where the benefit is believed to outweigh the risk," said Janet Woodcock, the director of FDA's Center for Drug Evaluation and Research.

Erin Gardiner, a Baxter spokeswoman, which makes an injectable form of metoclopramide, said the product has been safely and effectively used for more than 30 years and that tardive dyskinsia is a "rare" side effect. However, she said, Baxter would add the boxed warning to Reglan's label and provide patients with a medication guide that outlines the risks and benefits of the drug.

Metoclopramide works by speeding up the movement of the stomach muscles and increases the rate at which the stomach empties into the intestines. It is used as a short-term treatment of gastroesophageal reflux disease in patients who have not responded to other therapies, and to treat diabetic gastroparesis, the FDA said. It's recommended that treatment not exceed three months.

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; jennifer.corbett@dowjones.com