The U.S. Food and Drug Administration approved a once-monthly injection for multiple sclerosis from Biogen Inc and AbbVie Inc, with a boxed warning.

The self-administered, under-the-skin injection, Zinbryta, is designed to treat adults with relapsing forms of the disease.

Zinbryta should generally be used in patients who have not benefited from at least two prior therapies as the drug poses serious safety risks, the FDA said.

Accordingly, the approval comes with a boxed warning - the most severe issued by the FDA - to underscore risks, including liver injury and immune conditions.

Zinbryta, or daclizumab, was recommended for approval by European regulators in April.

MS is a chronic, inflammatory, autoimmune disease of the central nervous system that disrupts communication between the brain and other parts of the body.

The drug's effectiveness was shown in two clinical trials, the FDA said.

In one trial, patients were given either Zinbryta or Biogen's Avonex. Data showed that those on Zinbryta had fewer clinical relapses.

The second trial compared Zinbryta against a placebo, with data again showing that those on Zinbryta had fewer relapses.