Federal regulators are requiring more study of a proposed Alzheimer’s treatment before it can be approved for the public.

The Federal Drug Administration informed Acadia Pharmaceuticals in August that the company must conduct more clinical trials before the agency will consider approving the use of a drug called pimavanserin to treat some of the symptoms of Alzheimer’s disease.

Primavanserin, which Acadia markets by the name of Nuplazid, was approved in 2016 to treat hallucinations and delusions that are experienced in Parkinson’s disease. There are no authorized drugs for treating those same symptoms in Alzheimer’s, however.

In requiring more information about primavanserin’s possible benefits for Alzheimer’s patients, the FDA followed the recommendation of its Psychopharmacologic Drugs Advisory Committee. The committee criticized the scope of Acadia’s Alzheimer’s-related studies as being too small to adequately answer whether the drug can be effective in treating dementia psychosis.

The FDA should “work urgently” with Acadia to provide additional data needed to clear the drug for public use, said George Vradenburg, chairman of Us Against Alzheimer’s, an advocacy organization for Alzheimer’s patients and caregivers.

“It’s hard to imagine the fear and suffering when someone with Alzheimer’s experiences severe hallucinations or delusions and there are no treatments approved,” Vradenburg said,

Psychosis is included in up to a third of Alzheimer’s cases, according to Dr. Howard Fillit, M.D., chief science officer of the Alzheimer’s Drug Discovery Foundation.