Biohaven has hit another regulatory hurdle for its epilepsy candidate opakalim, piling fresh uncertainty onto the cash-strapped biotech.

Biohaven confirmed Thursday that the Food and Drug Administration had placed a partial hold on a clinical study for its epilepsy drug candidate, opakalim. Shares sank 14% to $12.87 on the news.

The drugmaker received a letter from regulators on Sept. 4 citing "insufficient information" regarding a specific metabolite found during rodent testing. Consequently, the FDA has paused new enrollment in the RISE-2 study evaluating the drug for focal onset epilepsy pending additional data. Currently enrolled participants will continue receiving treatment.

It is unclear how the metabolite, a small molecule produced when the body breaks down the drug, affects human subjects. Biohaven has pledged to submit additional lab testing.

To seek regulatory approval, Biohaven must finish the study as well as a separate late-stage study unaffected by the hold. Biohaven anticipates a data readout for that study, RISE-3, later this year, in what William Blair analyst Myles Minter calls a "key event" for the drugmaker.

Analysts were already working without a firm timeline for the RISE-2 data. While the enrollment pause will "no doubt incur a delay" to opakalim's potential New Drug Application, exact timing remains uncertain, Minter noted.

Last month, Biohaven entered a licensing agreement with South Korea's SK Biopharmaceuticals for opakalim. If the drug succeeds in clinical trials and gains regulatory approval, Biohaven will be eligible for milestone payments and sales royalties.

The biotech is based in New Haven, Conn., where it spun out of Yale University over a decade ago. Its most notable commercial successes were migraine treatments including Nurtec ODT, which secured regulatory approval in February 2020.

Pfizer acquired the company two years later, gaining control of its migraine and pain portfolio, while the experimental assets were spun out into a newly independent company-the current-day Biohaven.

However, the company's second chapter has been marked by turbulence. In November 2025, the drugmaker slashed its research and development budget by roughly 60% following the unexpected rejection of its rare disease drug, Vyglxia.

Other drug candidates, including one for spinal muscular atrophy, fell short in clinical trials. Opakalim itself failed to meet the primary endpoint of a depression study in late 2025. As Biohaven's cash reserves continue to dwindle, bringing a product to market is becoming an absolute necessity.

While the latest setback heaps further pressure on Biohaven and its pipeline, it is good news for Xenon Pharmaceuticals, which is in late-stage testing with its rival drug, azetukalner. Minter noted that a potential delay "widens Xenon's first-mover advantage" in epilepsy.