The history of autism "cures" is a history of dashed hopes and frequent danger.
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."
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Decades ago, the drug became a popular treatment for children with Fragile X syndrome, an inherited condition that affects a region of the X chromosome and is a leading cause of autism.
Until genetic tests for Fragile X arrived in the 1990s, scientists used a microscope to look for "fragile" or "broken" regions on the X chromosome. And they found that those abnormalities were easier to see in brain cells grown in a medium low in folic acid (a synthetic form of folate).
"So the very first, and most obvious theory was that Fragile X must have something to do with folic acid metabolism," says Dr. Michael Tranfaglia, medical director of the FRAXA Research Foundation and parent of an adult child who has the disorder and severe autism.
Parents started giving folic acid to their children with Fragile X. When that didn't work, they moved on to folinic acid — leucovorin.
"There was a lot of excitement about that, until people started doing actual clinical trials," Tranfaglia says. Then it became clear the drug was no better than a placebo.
Now, Tranfaglia says, leucovorin is back.
"It's not terribly surprising," he says, "because for every supplement and every vitamin you can possibly imagine, someone has proposed some kind of link to autism."
Usually, though, that someone is not running the FDA — the agency that determines whether a drug is safe and effective.
