WASHINGTON (Reuters) - Health officials have called off plans for a study examining a controversial type of treatment that some autism activists have touted as alternative medical therapy for children with the condition.

Families from across the U.S. living with autism take part in a rally calling to eliminate toxins from children's vaccines in Washington June 4, 2008. REUTERS/Yuri Gripas

The National Institute of Mental Health, or NIMH, part of the U.S. National Institutes of Health, said in a statement on Wednesday that it has canceled a study aimed at assessing the effectiveness of a treatment called chelation.

Chelation (pronounced key-LAY-shun) is a type of therapy in which a man-made amino acid, called EDTA, is added to the blood, and it has been used to treat heavy metal poisoning.

Some autism activists have advocated it on the theory -- rejected by most scientists -- that autism is triggered by exposure to mercury, a heavy metal, from childhood vaccines.

Many studies and medical experts have dismissed the notion that mercury used in a vaccine preservative causes autism, but some parents of autistic children strongly believe it does.

Since 2001, with the exception of some influenza vaccines, the mercury-containing preservative has not been used in routinely recommended childhood vaccines, according to the U.S. Centers for Disease Control and Prevention.

“NIMH has decided that resources are better directed at this time to testing other potential therapies for autism spectrum disorders, and is not pursuing the additional review required to begin the study,” the institute said.

The study was given initial permission to proceed in 2006, according to NIMH’s statement.

“No subjects were recruited for this clinical trial,” according to the statement.

“In February 2007, based on new scientific data, an NIH Institutional Review Board reassessed the risk-benefit ratio of the proposed study. The board determined that there was no clear evidence for direct benefit to the children who would participate in the chelation trial and that the study presents more than a minimal risk,” it added.

The institute said the only way that the study could go forward would be through an additional governmental approval process, and it decided not to pursue this.