Dementia expert Dr. Jason Karlawish told CNBC he’s skeptical of the Food and Drug Administration’s approval of Biogen‘s Alzheimer’s disease drug, Aduhelm, saying “the evidence to approve the drug wasn’t sufficient.”

“Another study is needed to establish whether this drug, in fact, is effective. Unfortunately, the FDA approved the drug for marketing, although they also do want another study,” the co-director of the Penn Memory Center at the University of Pennsylvania said on Monday following the agency’s formal OK.

The FDA’s approval marks the first new treatment for Alzheimer’s in nearly two decades. Alzheimer’s is a progressive neurodegenerative disorder that slowly destroys memory and thinking skills. More than 6 million Americans live with the disease, according to estimates by the Alzheimer’s Association. 

Karlawish told “The News with Shepard Smith” that there are a lot of promising Alzheimer’s drugs in the pipeline.  

“I’m optimistic about the coming future here, so I have hope. I just think this is not the drug upon which to pin our hopes,” he said. “Desperation should drive funding for Alzheimer’s research, it should not drive the interpretation of scientific evidence.”

Clinical trials found some patients who got the approved dose of Aduhelm experienced painful brain swelling.

“What you’re asking someone to do, is to take a chance at uncertain benefit, but known risk,” Karlawish said of prescribing the drug to patients.

The FDA said it will continue to monitor the drug as it reaches the U.S. market. The agency granted approval on the condition that Biogen conduct another clinical trial. 

Karlawish told host Shepard Smith that Biogen will face a challenge in “how to do that study when the drug is also available for clinical prescribing.” 

Representatives for Biogen and for the FDA did not immediately return requests for comment on Karlawish’s statements.