Hey,
Well this is what I've been told by various doctors, but not sure how much of it is true:
The pill doesn't actually make you gain weight, it increses your appetite, which can obviously lead to weight gain if you give in to the hunger (I've been on it for years now and think I definately ate more!)
The plus side to CD is that you should lose your appetite when in ketosis, and so shouldn't feel hungry anyway (even people like us on the pill). My CDC also said that CD doesn't affect the pill working in anyway (phew!).
Hope this helps