It probably won't hurt you TOO badly. Depends on a couple of factors.. for instance, how long ago you took the class. Unfortunately it will drop your Pharmcas GPA. Because some schools have strict cut off points for GPA, this might make you ineligible for those schools depending on what your GPA is now. But most will look at the bigger picture and how you did in the prerequisite classes and other classes that are more relevant to pharmacy. I had quite a few F's, D's, and W's on my transcript but most were from 8-12 years ago and were not in prerequisite classes. I have received interviews at 3 out of the 4 schools that I have applied to.
A quick note to everyone regarding retaking a class.. be careful if you're going to do that and only retake if you know you can get an A. I mention this because of the way that Pharmcas calculates GPA. If for example, you got a C the second time (after originally receiving an F), the C would not replace the F. It would just add to it. One might also be led to believe that getting a C would now give you a D average after being combined with the F. But this is not the case, as you would now have a C AND a F on your transcript. Even getting a B in the class would probably not help your cause. (Unless your GPA is below 3.00). For example, say you have a GPA of 3.15 (including the F). You might be tempted to think that the B will help pull up the F grade, but if you already had a 3.15 GPA, that B is just going to pull you down further. I only mention this b/c I got an F in an intro Bio class back in '98. I retook the class because I pretty much had to at the time, and I ended up getting a C. In retrospect, the C only ended up bringing down my GPA further.