h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

It should be noted that one of the more unusual effects of imbibing too much of this stuff (or Murphys or Beamish) is black poo. A bizarre and alarming phenomena to be sure, but something you should be prepared for when recovering from a heavy session.

A further note on this important point to calm the fears of any potential Guiness drinker. I was once ill for several weeks and had to stop drinking Guiness. After a time I was surprised to note that my poo returned to its normal colour. So the effects are not permanent.