Very, very briefly (I'm preparing a very long blog post on this, but I want to post it when Dr Hickey, my uncle, releases his book on this, which won't be for a while yet) - meta-analysis is essentially a method for magnifying the biases of the analyst. When collating the papers, nobody is blinded to anything so it's very, very easy to remove papers that the people doing the analysis disagree with (approx 1% or fewer of papers that turn up in initial searches end up getting used in most meta-analyses, and these are hand-picked). On top of this, many of them include additional unpublished (and therefore unreviewed) data from trials included in the analysis. You can easily see how this could cause problems, I'm sure. There are many, many problems of this nature. I'd strongly recommend everyone do what I did (for a paper analysing these problems) - go to the Cochrane or JAMA sites, and just read every meta-analysis published in a typical year, without any previous prejudice as to the worth or otherwise of the technique. If you can find a single one that appears to be good science, I'd be astonished...