If I remember correctly, a recent article pointed out that prescriptions in every category of drugs have increased year over year at a similar or greater rate, and that prescriptions for ADHD and ADD have actually been increasing slower than expected compared to other categories of drugs. An article like this should qualify the magnitude this way.

Secondly, as companies release new drugs that can target specific conditions better or safer than ever, treat conditions that were not treatable before, it makes sense that doctors prescribe medicines more and more often every year, especially as patents on proven drugs expire and the medications become available as cheap generics.