The Food and Drug Administration pushed back its target date for a decision on Zogenix’s epilepsy medicine Fintepla by three months to June 25, citing the need to review new information submitted by the company.Zogenix shares fell 18% to $21.36 in early trading amid concern about the future of the drug, designed to treat Dravet syndrome, a rare form of epilepsy that begins in infancy and can cause frequent and prolonged seizures.Zogenix sought to reassure investors about the medicine’s future. “We remain very confident in the data,” Zogenix CEO and President Stephen Farr said in the company’s release.

The success of Fintepla is crucial for Zogenix, which has only one other drug in clinical trials, MT1621, for a rare genetic disorder called thymidine kinase 2 deficiency. Analysts at Stifel forecast potential U.S. sales of more than $400 million for Fintepla in Dravet syndrome, with added potential for use in another severe form of epilepsy known as Lennox-Gastaut syndrome.

While the company didn’t give any details on the new information that triggered the delay, Stifel analysts said they heard from management that the request was about an efficacy analysis and wasn’t about safety or an analysis of whether the Phase 3 trials were successful.

Still, Zogenix’s stock drop reflects the nerves of shareholders, who have already seen several stumbles with the medicine.

Last year, the FDA initially refused to accept Zogenix’s application for Fintepla, citing two problems with the submission. The company reapplied in September and in November announced that the agency had given the drug a six-month priority review.

Then earlier this month, positive results from a study of Fintepla for Lennox-Gastaut syndrome failed to live up to expectations of some on Wall Street, causing another stock drop. Before the announcement, Zogenix shares had closed at $52.62 on Feb. 6.

Any concern about fenfluramine is heightened by its history. Fenfluramine was widely used in combination with phentermine in an off-label diet treatment known as Fen-Phen before reports of cardiovascular issues forced its withdrawal in 1997.

The dose that Zogenix uses with fenfluramine is significantly lower than what was used for weight loss, however, and no signs of cardiac toxicity have been observed in clinical study of Fintepla, according to the company.