The U.S. Food and Drug Administration on Wednesday warned about the risk of a rare but serious allergic reaction with the use of multiple sclerosis drugs including Teva's Copaxone.

The health regulator is adding a new boxed warning, the FDA's most prominent warning, to include information that a serious allergic reaction called anaphylaxis can occur at any time, from as early as after the first dose or following doses administered years after starting the medicine.

The drug, glatiramer acetate, is approved to treat patients with relapsing forms of multiple sclerosis, a chronic disease that affects the central nervous system.

Teva markets the drug under the name Copaxone, while a generic version of the drug, Glatopa, is sold by Sandoz.

The companies did not immediately respond to requests for comment.