It is pretty impressive to see how many medications patients take in the U.S. I looked at the official statistics from CDC and my anecdotal observations were confirmed. From 2011 to 2014 approximately 49% of population used at least one prescription drug in the last 30 days. In the same period approximately 12% of population used five or more prescription drugs in the last 30 days. All this started with the antibiotics, the first real successful class of medications. Then, the medical system tried to apply the same principles to treat all human diseases. It was successful for some diseases (e.g. diabetes, high blood pressure, hypothyroidism) but for other diseases the success was not that great (e.g. cancer, pain). One major difference is that now patients are on medications for years compared with the short therapy with the antibiotics. Long term use of medications comes with side effects.
When you take a medication the goal is for the active chemical to get to a target in the body (usually a receptor or enzyme) and alter an intracellular pathway. The reality is much messier than the textbooks. Nature used the same building blocks many times to create different receptors in the body. Medications are very rarely (maybe never) specific to just one target. One interesting website that shows possible interactions for medications is: http://stitch.embl.de/. Human receptors and the intracellular pathways can also have genetic variations and result in different effects for certain patients. Also, the medications are usually prescribed for one organ disease, but the molecular targets are present in other organs. One example of effects on other organs is the side effects of NSAIDs (nonsteroidal anti-inflammatory drugs) on gastrointestinal tract, cardiovascular system and kidneys.
The side effects to medications range from something quantifiable (e.g. changes in blood glucose, white blood cell count) to something very vague (e.g. feeling tired). One website that lists side effect to medication is: http://sideeffects.embl.de/. The problem with the vague side effects is that although they can be very bothersome, sometimes they are not caused by the medication. Most people heard about placebo effect. However, there is the opposite effect, the nocebo effect. Basically, you give a "sugar" pill to somebody and the subject complains of random side effects. Normal people on no medication walk around feeling tired, sleepy, maybe slightly nauseated once in a while. When we give them a pill (active or inactive) they start to attribute their usual feelings to the pill. This is actually a big problem, there are clinical studies showing that 1 in 5 people receiving inactive pills complain of side effects (Barsky et al 2002).
When patients take more than one medication, the interactions are very unpredictable. I watched an intriguing TED talk by Russ Altman. He describes how they found that pravastatin and paroxetine when taken together increase blood glucose. Innovatively, they used a bioinformatics approach and a side effect database. Knowing the mechanism of action of pravastatin (a cholesterol lowering drug) and paroxetine (an antidepressant) the blood glucose effect is very surprising.
What can we do better in the future? We should think twice before prescribing medications long term. Sometimes there are other options, like healthy diet for weight loss (which helps diabetes, high blood pressure) or relaxation/meditation techniques for anxiety and sleep. Only if everything fails we should rely on medications. Or we can use medications only short term while implementing the other methods. Our society is looking for a quick easy fix for every health problem, but the long lasting fix usually takes time and requires effort.