This is why we replicate studies. Unreplicated parapsychology results are a dime a dozen.
The whole field of systematic reviews and meta-analyses has developed around the need to aggregate results from multiple studies of the same disease or treatment, because you can't just trust one isolated result -- it's probably wrong.
http://en.wikipedia.org/wiki/Meta-analyses
Statisticians working in EBM have developed techniques for detecting the 'file-drawer problem' of unpublished negative studies, and correcting for multiple tests (data-dredging). Other fields have a lot to learn...
Regardless of the true reason, these are never carried out before a new drug or treatment is approved (because there is usually one or two studies supporting said treatment, both positive).
And if you have pointers to techniques developed for/by EBM practitioners, I would be grateful. Being a bayesian guy myself and having spent some time reading Lancet, NEMJ and BMJ papers, I'm so far unimpressed, to say the least.