GlaxoSmithKline Plc (GSK)’s anti-seizure drug Potiga now carries a black-box warning in the U.S. on risks including potential vision loss.

The warning, the most serious type the U.S. Food and Drug Administration issues, underscores risks of abnormalities in the eye, vision loss and skin discoloration, all of which may become permanent, the FDA said in a statement today. The revised label comes after the risks were flagged by the FDA in April.

Patients should have eye exams before starting Potiga and every six months during treatment, the FDA said. The drug accounted for 7 million pounds ($11 million) of the London-based company’s sales last year and is projected to bring in 39.5 million pounds this year, according to analyst estimates compiled by Bloomberg.

“In light of these reported adverse events, we have worked closely with regulators to update the medicine’s labeling to restrict its use to those patients where other appropriate medicine combinations have proved inadequate or have not been tolerated,” Glaxo said in an e-mailed statement today. “We review the safety of all our medicines on an ongoing basis.”