Can Buddhism be harmful? There is the much talked about example of "zen sickness".

But I wondered can any buddhist theory or practice can ever be harmful to a human?

I'm looking for an answer from anyone, not necessarily a Buddhist perspective. Though, of course, I'd be dismissive of anything unreasonable.
i can say that Buddha was mindful of every word he said making sure they are beneficial to all beings.  However, there might be some thing that seems to be harmful in short term but actually good in long run.  For example, during Buddha's time, 500 monks committed suicide after contemplating unpleasantness of body and 5 skandha.  (Buddha often talked about the drawbacks of all things that create sensual pleasures).  Those monks took it over the top and felt disgusted by their conditions so they took the wrong way out.  Later, someone asked Buddha if he didn't see that coming, Buddha said those monks would have taken their lives anyway from past karma.  at least they got some true knowledge in dhamma while they were still here which would benefit them in the future. 
So in short,  practicing Buddhism according to Buddha (8 folds path) is not harmful.