http://www.cs.unc.edu/~azuma/hitch4.html
This was the same piece I read before I decided to apply. You absolutely
NEED a positive reason for doing any sort of graduate study, mainly because the $ isn't worth it to go for graduate school unless it is for the sorts of jobs or pursuits that the PhD is the union card for: teaching (which I can say doesn't pay), industry work (which I can say does pay but is inherently unstable), government (Apt Quote: "The civil service pretends to pay about the amount we pretend to work"). Don't do it as a "I don't know what to do" kind of pursuit. I would easily say that pursuit costed me between $400k and $600k in lost earning potential. I actually made it back, but that is the exception and that was not my intention in the first place. There's a longer talk involved if you are interested, and it has to do with the classical graduate programs that are considered good at training (Wisc and UMN are well-known for faculty and "leadership" training in all the programs, Purdue, Rutgers, UNC, and Michigan are well-regarded industrial pharmacy programs; Texas, Kansas, Florida, and Pitt have excellent med chem, etc., etc....).
Also, the math backgrounds needed for graduate pharmacokinetics/pharmacometrics is almost certainly now at the Partial Differential level and preferably with a class in numerical methods (far higher than the Pharmacy school prerequisites). Preferable background with some class in Physical Chemistry. Recommend that you go to the library and pick up the Bonate or the Gabrielsson or the Ette book to see what you're dealing with.
The math background for pharmacoeconomics/HTA/outcomes can be less, but the statistics is a bit more exacting. Depending on matters, the Mickey major version only needs calculus and the ultra hardcore version needs Real Analysis and Measure Theory to take on Shao's Mathematical Statistics. Programs are migrating more to the hardcore version as the methods competition with FDA keeps getting more heated. The Piantadosi Clinical Trials and the Shao Mathematical Statistics is probably what you want to look at for a sense of where you ought to be at the end of the training for the advanced. There's not really a great Health Econ textbook at the moment, the Handbooks are generally used. The readings for this class are credible and are more likely in the majors class rather than a textbook:
https://canvas.uw.edu/courses/949881
Try lecture 2A's reading for a background on basically what you end up dealing with in the pharmacoecon/HTA program.
The reason why I talk about the math backgrounds is that it is usually that which would limit or preclude taking any of the really fun classes and puts a clear ceiling on what is possible to accomplish in 3-4 years.