Possibly more damaging than an unrelated course but definitely less damaging than a prereq, assuming that "bad grade" means a 'C'. If you actually fail a course, I don't imagine schools would look favorably on that, but I don't know the weight of the consequences either. Your overall math and science GPA (one of the GPAs that PTCAS calculates) may take a minor hit, but you should be just fine as long as you did well elsewhere. Isn't it a given that everyone struggles with ochem anyways?