In a previous life, I worked with mostly medicated kids in a clinical K-12 setting. It was absolutely the norm for them to be inconsistent with their meds.
I've been told that the segment of people on meds for psychological disorders who go off their meds when they shouldn't, at some point in their treatment, approaches 100 percent. (And when I say "when they shouldn't have," I mean the solution for the problems that inevitably arise ends up being to get back on the meds, or similar ones.)
I could almost believe that. Most of the drugs are still in the "we think this is how they work" category. You have a psychological disturbance that results in paranoia (which can and does happen to people even with no diagnosed illness or even on medication), and the medication is an easy thing to lash out at. Or you experience tons of the listed side effects (either real or imagined, it wouldn't matter) and can't convince a doctor to change the medicine. The latter happened to me, real side effect was losing memory; found notes that I had told the doctor several times over a year, and he did nothing. I called their 'emergency assist' phone, left a message that I would stop unless I heard back from them. Never did, so I went cold turkey and switched doctors after the weekend was over.
Had mine been for anything other than pain and depression and insomnia, that withdrawal could have been hilariously amusing; instead I just sat up reading a book for over 48 hours til I passed out.
But my depression is a strange one; mild sufferers (by the DSM meaning of mild depression or any other illness or axis) of illnesses with no Axis 1 or 2 components who doesn't suffer from delusions aren't likely to stop taking meds that work. Additionally, barring a massive incident, most non-psychosis and non-paranoia disorder sufferers are very likely to stay on a med that works; without something that alters perception of reality, they have no reason to go back to the pain and suffering of before. Incidents like moving (the wait list for a psychiatrist here was over a year!), insurance covering a different doctor, losing a job/house/etc, that are outside the individuals' control shouldn't be counted.