I can't tell you how many times I have seen patients on 10...20...30! different meds when I worked in nursing homes! This is far more common than you might think and often the medications are coming from different specialists who are unaware of what the other doctors are prescribing. And the problem is the side effects are not just those listed as reverse reactions listed under the individual drugs and then added up. When someone is taking numerous drugs completely new compounds are being formed within their body....and no physician or pharmacist can tell you what the hell is going on!
I actually found it amazing that some of these folks could function at all with all the chemicals that were being pumped into them...one of many things that has amazed me about how resilient the human body really is!
For anyone interested...click the link below to find out how two or more drugs interact. This probably won't give you the whole picture when the drugs gets into the double-digits...but it's a start.