Why did they just now realize that chelation with DMSA is dangerous, and should not be used on children who do not have lead poisoning*? According to this article, the results of this 2007 study convinced a NIMH review board that the risks of giving DMSA to children with normal blood and tissue lead levels were greater than they had previously thought. The study, done with rats, showed that exposure to DMSA in rats without any previous exposure to lead impaired those rats' performance on tests of attention and learning.
From the study's discussion of the results:
The present study also revealed the unexpected finding that a single 3-week course of succimer treatment during early development produced lasting dysfunction in cognition and arousal regulation in rats not previously exposed to Pb. Note that these four behavioral tests were administered across a 7-month period following cessation of succimer treatment, suggesting lasting brain changes. ... These various impairments were similar in magnitude to those produced by the High-Pb exposure. (Emphasis mine).
Faced with this new information, NIMH decided to cancel the study and instead use its funding to pursue other autism-related research topics.
While I am gratified that the NIH decided not to go forward with either of these irresponsible investigations, I remain puzzled as to why they even approved them in the first place. In the case of the autism study, there is already a huge body of evidence showing no relation between heavy-metal poisoning and autism, and in the case of the TACT, the medication being studied was contraindicated for the category of patient making up the entire study population! And this information didn't even make it into the consent form. They should have quashed these studies while they were still in the planning stage.
*Yes, lead, not mercury. While there are a few cases of DMSA being used to treat mercury and arsenic poisoning, they do not constitute sufficient evidence that it is safe and effective for use with those metals. Consequently, DMSA has only been FDA-approved to treat lead poisoning.