Although you only need approximately two years of undergraduate course work to be admitted to pharmacy programs, many people choose to finish their bachelor's degree first. Or, many programs award a bachelor's degree in pharmaceutical sciences (BSPS) while you pursue your Pharm.D.
Most schools encourage you to enroll in the pre-pharmacy program early. This allows you to fulfill your school's general education requirements while also taking pre-professional math and science courses required by the pharmacy program.