The FDA will soon decide the fate of Biogen's experimental Alzheimer's drug. But there is one glaring issue — there is no conclusive evidence the drug effectively treats the crippling neurological disease.

Why it matters: This will be one of the FDA's most important decisions in years. The outcome will show whether the federal agency sides with the overwhelming scientific consensus that the drug isn't proven to work, or with an industry and a patient population desperate for anything to be approved.

The background: Two years ago, Biogen halted two clinical trials of its infused drug, aducanumab.

• But the company looked again at the clinical trial data and argued one of the failed trials suggested some benefit for Alzheimer's patients on a higher dose of the drug. The other trial still failed.
• The FDA generally wants "two adequate and well-controlled trials" for drug approval, although it does on rare occasions consider one well-controlled trial with "confirmatory evidence."
• Biogen applied for FDA approval last summer.

Where things stand: Independent physicians, statisticians, drug researchers and others in the field have urged the FDA to reject the drug, warning the study supporting aducanumab is small, flawed and possibly a false positive. Side effects like brain swelling have also raised concerns.

• The most notable dissent came during the FDA's advisory committee meeting last November, when none of the 11 members voted in favor of the drug's approval.
• "What's the point of an advisory committee if you approve this? The FDA just doesn't consider your opinion at all," said Brian Skorney, a pharmaceutical stock analyst at Baird who added it would be a "huge misstep for the agency" to approve the drug.
• The FDA's statistical reviewer of aducanumab said there was "no compelling substantial evidence of treatment effect or disease slowing and that another study is needed."
• David Knopman, a Mayo Clinic neurologist who oversaw the aducanumab trials, previously said the drug's evidence was "terribly weak." He told Axios he believes Biogen needs to run another clinical trial, and the company could have been close to finishing if it had done so two years ago.

The other side: Biogen did not make executives available for an interview and would not answer questions on the record. The company's market value will plummet if aducanumab doesn't get approval.

• Many physicians and scientists who support aducanumab's approval have financialtiesto Biogenand Eisai, which also developed the drug.
• Alzheimer's patients who tried the drug, and their families, are looking for any kind of treatment that stops the disease's progression and broadly want the FDA to approve the drug despite the dearth of evidence.
• "There's a dire and drastic need to offer relief," said Joanne Pike, a top executive at the Alzheimer's Association. She said the organization "respectfully disagrees" with the FDA's expert committee. When asked if Biogen should have started another trial two years ago or if Biogen should start another trial if the drug isn't approved, she said, "I'm not going to comment on that."

The bottom line: "The FDA's credibility would take an enormous hit if this product is approved," Caleb Alexander, a physician and drug researcher at Johns Hopkins who was on the FDA's advisory committee for this drug, told Axios. "It's important they get it right. There's no reason they should follow the science any less here."