that is always a good point, and you can never REALLY know for sure, however the fact is roughly 500,000 people take ecstasy every weekend and there is not enough bad statistics for it to be something that can be up for serious consideration.
obviously im guessing here but as you dont hear about it that often (and when it does you can bet your bottom dollar that the red tops will use it for front page) that its probably just as likely to get a bad pill as it is to get an alcoholic drink thats gone off!
but as we dont know enough 'true' statistics about the illegal drugs i admit that it is pure speculation on my part!