Some medications act backwards - they cause the problem they are prescribed to prevent. So I think of them as backwards. A recent example of this is the drugs prescribed to prevent osteoporosis rarely cause degeneration of the jawbone or fractures of the thigh bone.
In a nutshell, here is how my non-medical brain understands this: Drugs are okayed through clinical trials. There is increasing pressure to decrease the length of time of clinical trials to speed drugs to market (which is another post all together) - now I think of them as taking around 3-5 years of human tests. But then we have an aging population who is living longer with chronic conditions. Take the two together and we have populations who are taking prescriptions, or even OTC medications, for five times or more the length of the clinical trials. New long term side effects are then discovered.
Now I have been on synthetic thyroid hormones for nearly thirty years. I seem to be doing okay. There are issues reported with bone loss if you are on it for long term. So if I live to be 80, I will have been on it for sixty years. Do they have any studies for people who are on it for that long? Probably not.
This is another issue in modern medicine. Aging populations who are on medications for decades. What about these really long term patients and their side effects? Another factor in the problems of redesigning clinical trials.