If I write a post and take a link about illegal pharmacy related product, will it danger for my PR and search engine index?
I doubt it, there lots of crap like that on sites that are still indexed. Unless you talking about linking out TO a site that could be considered a "bad neighbourhood"
Gogoel rade pages not only based upon their copy and inbound links, but also upon the pages that they link to. So watch out, google has already that it hates bad neighbours