Why are there so many ADRs?  There are many reasons.  Here are just a few.
First, more drugs -- and many more combinations of drugs -- are being used to treat patients than ever before.  To exemplify this point, 64%  of all patient visits to physicians result in prescriptions. {Schappert}
Secondly, 2.8 billion prescriptions were  filled in the year 2000.{National Association of Chain Drug Stores} That is about 10 prescriptions for every person in the United States.
Finally,  the rate of ADRs increases exponentially after a patient is on 4 or more medications.{Jacubeit}
Efforts to reduce polypharmacy are important but for many patients, the number of medications cannot always be reduced without doing harm.  That is why it is important to understand the basis for drug interactions.  This will allow us to make the most appropriate choices in prescribing and avoiding preventable ADRs.

Schappert SM. Ambulatory care visits to physician offices, hospital outpaitent departments, and emergency departments: United States, 1997. National Center for Health Statistics. Vital Health Stat. 13(143). 1999.

National Association of Chain Drug Stores. 2000 community pharmacy results.  2001. Alexandria, VA.

Jacubeit T, Drisch D, Weber E. Risk factors as reflected by an intensive drug monitoring system. Agents Actions 1990; 29:117-125.