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.

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.

For instance officials at the Food and Drug Administration are enforcing a serious warning on a heartburn drug called Reglan, stating that its use may be linked to muscle spasms. These problems, including uncontrollable movement of the limbs, face and tongue, are usually irreversible even after patients stop taking the drug, according to the warning, cited by the Associated Press.

The FDA warns that since such symptoms of movement disorder are not often reversible, the maximum duration of the treatment should be three months because the inherent risk is "directly related to the length of time a patient is taking metoclopramide and the number of doses taken."

More than 2 million Americans use the medicines to treat gastroesophageal reflux disease and other stomach ailments. Therefore the agency ordered manufacturers to add a "black box" warning, the strongest available, and develop a patient-friendly guide explaining the risk.

Tardive dyskinesia, which has also been associated with many drugs used to treat schizophrenia and bipolar disorder, is rarely reversible once it becomes obvious.