BRANFORD, Conn. – Biotechnology company Neurogen Corp. said Sunday it stopped a clinical trial of its insomnia candidate adipiplon because of negative side effects.

The Branford, Conn. company is developing immediate-release and controlled-release forms of the drug, and was testing a tablet that combined both versions. Neurogen said the combined drug may not be working as expected, leading to “a higher than anticipated rate of unwanted next day effects” in the phase 2/3 trial.

Patients have received both kinds of adipiplon in previous trials, but this is the first trial of the combination tablet. Neurogen said it will investigate before continuing with the trial.

“We are disappointed by this setback,” said Stephen R. Davis, president and chief executive of Neurogen, in a statement. “We do not yet know whether there is a path forward with lower doses of the existing formulation or whether further formulation development would be required.”

The company scheduled a conference call to discuss the results at 8:30 a.m Eastern time.