Why Is Mixing Prescription Drugs Deadly?

Many people have to take a bunch of prescription drugs, but how do doctors test drugs interactions to keep them from killing you?

Drug Interactions (
"The classifications below are a general guideline only. It is difficult to determine the relevance of a particular drug interaction to any individual given the large number of variables."

Discovering New Drug-Drug Interactions by Text-Mining the Biomedical Literature (Stanford)
"Americans are living longer than ever before, and with that increased age comes a greater reliance on pharmaceuticals. For example, recent estimates by Kaiser Permanente indicate that the average 70-year-old Americans over 30 prescriptions per year."