First, realize that many schools will not allow you to retake if you get a C or higher. If you had taken the class P/F, a C would mean you passed the class and that would make you ineligible. A C- typically is seen as a "failing" grade. Some schools require C's or higher in order to take the second step of a sequenced courses (ie OChem 1 to OChem 2)
That said, even if you are able to take the class again, you have to wonder what the benefit will actually be. Unless you completely bombed it or have to get a certain grade to take another required class, I think it's usually an awful idea to retake a class.
Let's say you get a straight C with a 74.2% in the class - passing, but obviously lower than you want. You, being a nervous pre-med decide to take it over again...what are the possible outcomes:
1) You ace the class, get and A+...and adcoms look at the grade and say "well, yeah...they took the class a second time, what type of idiot can't get something the second time through?"
2) You do better than previously, but it's a really hard class and you still only get a B+...and adcoms look at the grade and say "WTF, they retook it and still couldn't get an A?"
3) Heaven forbid you do the same or even worse, you might as well give up and start thinking about law school...they tend to be very impressed when anyone takes anything harder than history or polisci. Patent lawyers make a lot of money too and have some stringent requirements that undergrad majors be scientific in nature...maybe that's your best chance.
Options 1 and 2 are not good, the improvement of your AMCAS GPA is marginal, and the impressions of taking a class over again, probably nullify any gains made. There's simply nothing good that comes out of retaking a marginal grade. Getting an F or being in a situation where you can't take a required course further on down the line are obviously different and necessitate retaking, but otherwise, it's really not worth the time and effort.
