There's also the issue of interactions between various medications. Someone may consult a clinical practitioner who prescribes a certain medication, and then that person consults a different clinical practitioner who prescribes another medication for something else. Communication between clinical practitioners must be very open and very clear to ensure that there are no negative interactions.
There's something else. Side effects are one of the main reasons why people don't take prescribed medications. For young male military personnel who are given an anti-depressant for post-traumatic stress, depression or a sleep disorder, the sexual side effects of such a drug will be the primary reason they don't want to take it. If they do take it, they do so very irregularly, when it won't interfere with the activities they have planned.