A recently-approved Alzheimer’s drug had to overcome the objections of Food and Drug Administration (FDA) statisticians who weren’t convinced it was effective, according to newly-released internal memos.

The FDA granted “accelerated approval” to Biogen’s drug Aduhelm, the first new treatment approved for Alzheimer’s since 2003. The drug is thought by some to be the first therapy that attacks the neurological disease at the source rather than simply managing symptoms, but outside observers pushed back on the approval, citing doubts that Aduhelm was actually effective.

The now-public FDA documents show that FDA officials convened in the weeks leading up to the approval to devise a plan for getting the drug approved. Aduhelm was ultimately approved not for its exhibited effect on Alzheimer’s, which evidence is mixed on, but for its ability to remove beta-amyloid brain plaques, which are thought to contribute to the disease.

More than one million Americans are thought to be eligible for Aduhelm, which will run at a cost of $56,000 per year. Dr. Aaron Kesselheim resigned from the FDA’s advisory committee overseeing the drug’s approval, calling the process “the worst drug approval in U.S. history.” (RELATED: Clinical Coordinator Charged With Totally Falsifying Drug Trials, Data)

An FDA adviser who resigned in protest of an Alzheimer’s drug approval says he’s concerned about the agency’s standards.

First on @CBSThisMorning: @DrTaraNarula spoke with Dr. Kesselheim about why he resigned. pic.twitter.com/ReECvTkwSL

— CBS This Morning (@CBSThisMorning) June 21, 2021

The director of the FDA’s drug division Patrizia Cavazzoni pushed back on Kesselheim’s claim in an interview with STAT News on Tuesday. “We think the decision is on very solid ground, that we are on very solid ground when it comes to the data and the rationale for utilizing accelerated approval to greenlight this drug,” she said.

Outside advisers recommended against approving Aduhelm in November, sparking months of internal debate at the agency over how to proceed. One clinical trial on the drug showed positive results, another negative. Ultimately, the agency proved willing to take a chance on the drug that could help fight a debilitating disease that has seen little in the way of new treatments in recent history.

The FDA argued that it was “reasonably likely” that by fighting the beta-amyloid plaques, Aduhelm would result in a positive cognitive benefit for patients. This approval process broke agency precedent, as no drug had ever been granted accelerated approval due to mixed Phase 3 trials.

In one memo, FDA officials argue “the circumstances here are fundamentally different” and as a result, this approval won’t lead to drug companies abusing the accelerated approval process in the future. They also claim there is enough data to suggest the drug is beneficial that companies which stop and start failed trials won’t be able to gain approval in the same way as Biogen did.