You'd have to get a BS in Nursing. The prerequisites for pharmacy and nursing can vary though; you need to ensure you pick a school where the prerequisites for nursing are the same as the pre-med or pre-pharmacy courses (many nursing schools are light on the sciences; a few are not). There may be a few additional classes you'd have to take, too.
Of course, the two people above also mentioned important aspects of this plan as well: you'd need additional nursing experience to be admitted to a nurse anesthetist program. Of course, if you get a BSN, you can also work at the salary of a nurse while in pharmacy school (if you wind up doing that instead), which would be higher than the salary of a pharmacy intern, usually.