If I were you, I would take pharmacology. The course is difficult because of all of the concepts that are incorporated (e.g. biochem,physiology, anatomy, cell biology, and micro).
Given the intensity of those courses, you would be well advised to seize the advantage of taking the course before dental school. An undergrad version of pharm, may not be as tough as a dental school version, but any introduction to the core concepts of pharmcokinetics and pharmacodynamics is a plus. Without those concepts locked down, pharm is nearly impossible.
Take the class and thank God your school has the wisdom and knowledge to offer it.