Pre-med isn't a major, it's just an emphasis of courses you need to take to get into med school. You can be pre-med AND a chemistry major. It just means that in addition to your chemistry major requirements, you're taking all your medical school admissions requirements. Most of those are going to be included within a chemistry major anyway (i.e., 2 semesters of Gen. Chem with lab, 2 semesters Org. Chem with lab, 2 semesters Gen. Physics with lab), and you'd just have to add in the Gen. Biology and Genetics/Biochem requirements.
You have to take the courses they require for admission. Beyond that, you can major in anything you want. For a while, students were told that med schools liked to see more variety in their applicants, like English majors, etc. However, students who ONLY took the basic pre-med requirements and no other science curriculum have wound up struggling in med school, and that's no longer recommended as a good way to stand out as a unique applicant. You certainly could double major or have a minor in something like English, and be a more appealing applicant...that gets at what gravenewworld mentioned...that they look for well-rounded applicants. Medicine isn't just science, you have to deal with people and social issues, highly charged ethical issues, and just generally be able to relate to the wide variety of backgrounds your patients come in with. For this reason, courses in subjects like psychology, social sciences, study of other cultures (often incorporated into a foreign language curriculum), ethics (often within philosophy courses), improvement of communication skills (English expository writing courses, or communications courses) are all favorable supplements.
Other things that can help your application are shadowing or volunteer experiences in a medical setting, and research experiences (careful, you might wind up like I did and end up enjoying the research experience so much you withdraw applications from med school and enter grad school). These don't all have to be volunteer work. If you can get paid for the experience, there's nothing wrong with that. So, when you're thinking about summer jobs, for example, you might want to look into paid internships in research labs, or work at a hospital or doctor's office rather than heading to the nearest fast-food place or big box store for a job.
