If you have congestion and an upset stomach, would you treat the symptoms with two different drugs? And if you had a headache before your 10am meeting, would you wash down some acetaminophens with your morning coffee? Also, if two drugs claim to do the same thing, would taking both drugs together enhance or speed up the effects? In all scenarios, you would be ingesting two different drugs that could potentially interact with each other to produce unintended consequences.
Why is it dangerous to mix drugs? Each drug compound is designed to act on a particular biological pathway or component. Mixing drugs allows multiple pathways or components to be affected at the same time. This could produce effects that would otherwise not be seen if the drugs were not taken together. The effects could be overly potent, to the point of being harmful. On the other hand, the drugs could counteract with each other, negating any health benefits from either drug.
By the FDA's estimates, nearly half of Americans have used a prescription drug in the past 30 days. So a whole lot of people are taking drugs, and perhaps have been mixing drugs without even knowing it. That's why it's more important now than ever to double check with your doctor/pharmacist and read every label before taking a drug.