I'm sure there are a lot of options. Certainly hospital epi would be a start. Also within epi, I've taken two classes (pharmacoepidemiology and outcomes research) that are heavy on the pharm. We are several faculty who have appts in both epidemiology and the pharm school.
Outside of epidemiology, there would be plenty of other options. Some do research on cost of pharm treatments, or health services research, or patient behavior related to medication use. I didn't realize this until I started meeting folks in the pharm school, but they learn a lot more about health behavior, health education, and counseling than we did in med school. And, you could definitely focus on these areas within an MPH program.
Edit: Oh yeah. An obvious option would be clinical trials, too. Some MPH programs have tracks within epi or biostatistics about clinical investigation. But, all new medications need to be conducted by people who really know their stuff when it comes to drug trials.