It definitely won't hurt your chances. I was a post-bacc premed student and, because I already had my bachelors degree, I had a year off while I was applying to medical school. Because I've always enjoyed gerontology, I decided to spend that free year getting an accelerated masters degree (3 semesters). Fortuantely my application numbers were pretty much on target so I didn't NEED to do it, but I WANTED to do it (I enjoy school). But when I went to my medical school interviews, I was always asked about my work in graduate school and if I intended to complete my degree. When they asked why I went, I told them that I had always wanted to go to graduate school and knew this was the only time I would have do so, so I went.
So I'd say go for it if you enjoy the material you are going to be studying. Just be sure you pick something that you really enjoy studying (don't feel like it has to be directly medically related). I had a blast in graduate school, and don't have any regrets about doing what I did.
Good luck.