During chelation therapy, patients are given injections of a chemical that binds to heavy metals, lowering their concentration in the blood and ultimately allowing the metals to be excreted through urine.

Chelation gained traction as an alternative treatment for autism due to a theory that mercury poisoning might play a role in the developmental disorder. However, evidence hasn't supported that idea and it's been essentially discarded in the scientific community, researchers said.

The procedure also carries safety concerns, including risks of kidney damage and gastrointestinal problems.

Lead researcher Tonya Davis from Baylor University in Waco, Texas, said the study team's goal was not to tell parents which treatments they should or shouldn't seek for their children.

"I see that they want to try everything, and they are well intentioned," she told Reuters Health.

"But there are risks involved with any treatment choice, and some of those risks are very serious. So far science does not support (chelation) as being an effective treatment, and that's a big risk to take when you have limited resources and limited time."

The Centers for Disease Control and Prevention estimates that one in 88 kids in the U.S. has an autism spectrum disorder.

Davis and her colleagues found five studies that tested the effects of chelation in kids with autism. Those studies each had between one and 41 children, from age three to 14.

Researchers had given the kids chelation therapy - sometimes along with vitamin supplements or other treatments - between one and 12 times a week for up to seven months. They used tests and questionnaires or anecdotal reports from parents to see how symptoms changed over time.

The study with only one child, a four-year-old boy, found chelation had positive effects on autism symptoms based on a parent report. The other four studies all showed mixed results, with some kids improving on some symptom measures.

However, none of the studies provided any certainty that those benefits were due to chelation itself, and not another treatment or just kids getting older, the researchers wrote in the journal Research in Autism Spectrum Disorders.

Davis said she and her colleagues were surprised to find so few studies measuring the effects of chelation, given how many families they each knew that were using it. That lack of evidence was a concern, she said, along with the questionable study designs and conclusions.

"I just hope that parents get as much information as they can" before trying a new treatment, Davis said.

A typical package of chelation treatments runs for about $2,000 to $5,000. In addition to treating lead poisoning, chelation has also been used for cancer and heart disease.

But when it comes to autism, even calling chelation an alternative therapy is a stretch, said one autism researcher not involved in the new study.

"There's really no evidence that mercury causes autism or has a place in causing autism, and also we know that chelation can be dangerous as well. Even the underlying theories don't make sense," said Dr. Joyce Mauk, head of the Child Study Center, an organization that treats kids with developmental disabilities in Fort Worth, Texas.

"Most children with developmental disabilities, what gets them better is a really skilled therapist and lots of work," Mauk told Reuters Health.

"If you hear about something when all you do is inject something or take a pill, it's unlikely to work."

SOURCE: Research in Autism Spectrum Disorders, January 2013.

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