Just make sure you finish the other pre-reqs (psychology, sociology, etc) and if then take an upper level science course and see how you fare. For some of the more competitive programs, there may be an expectance of taking upper level courses and performing well to show sustained competence. It may not be necessary if your other ECs and stats are up to snuff - though taking an extra course or two won't hurt you (provided it won't hurt your wallet too much)