This is a toughy. It's hard to find information that will specifically mention the horrible side effects of drugs by any "credible organization". The government and FDA censor almost everything out there. Most stuff gets thrown under the rug. Drug companies will do 5 studies and pick the study with the best results and then let the world know about it.
My source of information on the side effects of drugs come from http://www.newstarget.com
and http://www.mercola.com, two very popular sites that show the effects of natural supplements and synthetic drugs. So type in "zyrtec" in http://www.newstarget.com and read the articles. You will find backings and studies. I hope this answers your question.