Skeptics of the drug warn that if the FDA approves AMX0035, it would be repeating the same mistake it made last year when it cleared a controversial Alzheimer's drug that critics said had little data showing it worked. The last one was approved five years ago.

There are two FDA-approved therapies on the market but they have limited effectiveness. About 30,000 people in the United States have ALS, sometimes called 'Lou Gehrig's disease.' Another 6,000 are diagnosed every year. Nevertheless, ALS advocates hold out hope that the FDA will clear the treatment in the face of intense pressure from patients and their families. On Friday, the FDA repeated many of its earlier concerns about the drug's effectiveness, saying additional analyses submitted by the company were flawed and included no new information. The agency staff has expressed skepticism about the drug and its single small clinical trial, as have those same advisers who voted narrowly against recommending approval in late March.

On Wednesday, the FDA is holding a rare, second meeting with a panel of independent experts to discuss the treatment, and is expected to make a decision on approval by Sept. The degenerative disease typically paralyzes patients, robbing them of their ability to walk, talk and eventually breathe, often killing them within two to five years of diagnosis. The debate over the therapy is about to take center stage in the often-agonizing world of ALS, or amyotrophic lateral sclerosis, a fatal illness that destroys nerve cells in the brain and spinal cord.

Dead cells fatal falls map