When I did IB pharmacology a couple of years ago I found it fairly interesting and not too tough. Kind of nice to know how various drugs act on the body and it's fun to know what people's prescriptions etc. actually do and how they work. Quite a lot of learning to be done in terms of drug names though.
This does make revision pretty easy as (at least when I took it) you could put a small set of essay plans together that covered a good proportion of possible exam questions and just learn those. There was also a bit of data analysis (trace analysis questions as they are called) which isn't too hard. Practicals weren't too taxing but I seem to remember some were assessed.
Just noticed that you didn't take either cells or physiology at IA. That could be a bit of a handicap as there is a fair bit of molecular signalling and organ biology involved though they probably aren't essential. Does the course guidline list any pre-reqs? Alternatively go to the lectures for the first week or so to get an idea of if you think it'll be ok or not, you can always decide not to continue with it.
Hope that helps.