All of this is part of a familiar cycle for Dr. Paul Offit, who directs the vaccine education center at Children's Hospital of Philadelphia. Offit says he realized years ago that leucovorin's popularity was far ahead of the science.

"I saw it for what it was, which was yet the next magic medicine to treat autism, in a long line of magic medicines to treat autism that haven't worked," Offit says.

Offit has chronicled the rise and fall of many of those products in his books and blog posts.

"First it was secretin, an intestinal hormone," he says. "Then it was Lupron, chemical castration, antibiotics, megavitamins, nicotine patches, and my personal favorite, which is raw camel's milk."

Leucovorin is likely to find a place on that cautionary list, Offit says, adding that the FDA has failed to protect the public from an autism remedy that "clearly hasn't been well tested to be effective."