At the end of 2013, the U.S. Food and Drug Administration (FDA) warned that a relatively common seizure drug could cause a life-threatening drug reaction.

Onfi (clobazam) is a type of seizure medication approved for treating epilepsy and other types of seizure disorders. More specifically, Onfi is typically used with other seizure drugs to treat Lennox-Gastaut Syndrome, a life-threatening form of childhood epilepsy that can cause developmental problems.

It is also sometimes used to treat anxiety and psychiatric issues. In the case of Lennox-Gastaut Syndrome, Onfi is often prescribed when other seizure drugs alone are not enough to control the disorder. However, Onfi may cause serious autoimmune reactions like Stevens Johnson Syndrome in some patients.

According to a Drug Safety Announcement issued by the FDA in December 2013, the federal agency had received reports that patients taking Onfi had suffered from Stevens Johnson Syndrome, or SJS.

Stevens Johnson Syndrome is a life-threatening autoimmune disease where the immune system aggressively attacks the patient’s own skin. Stevens Johnson Syndrome causes a painful blistering rash with sores and ulcers that can cover the body, and may even damage the eyes and internal organs. SJS symptoms can be fatal.

The FDA stated that both Stevens Johnson Syndrome and a more severe form of the autoimmune reaction, called Toxic Epidermal Necrolysis (TEN), had been reported in patients taking Onfi. Toxic Epidermal Necrolysis symptoms are more severe than TEN, though there are borderline cases that overlap between the two autoimmune conditions. Hospitals often treat SJS and TEN in the burn wards of their intensive care units.

SJS and TEN are relatively rare, making them hard to study. But it is known that the majority of cases are triggered by unusual reactions to drugs.

The FDA warning advised the public that they had enough information to mandate changes in the labeling of Onfi to reflect the risk of SJS and TEN. The new FDA warning also advised patients taking Onfi to immediately seek medical attention if they develop a rash with blistering or peeling of the skin, sores in the mouth, or hives.

However, the FDA did not recommend that patients stop taking Onfi without talking to their doctor, but recommends that healthcare professionals change patients onto another drug. Discontinuing seizure drugs such as Onfi is tricky, since stopping the drug suddenly can cause “rebound” seizures.