A company that claims to have the first drug to slow mental decline from Alzheimer's disease made its case to scientists on Thursday, disclosing more data that may help explain why one study of the experimental medicine succeeded and another failed.

Excitement and skepticism have surrounded the drug, aducanumab, since its developers stopped studies earlier this year because it didn't seem to be working, then did a remarkable about-face in October and said new results suggest it was effective at a high dose.

Thursday's presentation at an Alzheimer's conference in San Diego convinced some experts that it deserves serious consideration, but major questions remain, and it's not clear whether the drug can or should win Food and Drug Administration (FDA) approval. Changes during the study and unusual analyses make the results hard to interpret.

"I don't see how you can conclude anything other than that another trial needs to be done," said the Mayo Clinic's Dr. David Knopman. He is on an FDA panel likely to review the drug, but won't participate because he was involved in one of the studies.

Aducanumab aims to help the body clear harmful plaques, or protein clumps, from the brain. Biogen, of Cambridge, Massachusetts, is developing it with a Japanese company, Eisai Co. Ltd.

The stakes are high for approval or denial. More than 5 million people in the United States and millions more worldwide have Alzheimer's. Current drugs only temporarily ease symptoms and do not slow the loss of memory and thinking skills.

But approving a drug that isn't truly effective could expose patients to financial and medical risks and give other drugmakers less incentive to develop better treatments.