Like others have said, it's all about what you are taking. If you need to take the basic pre reqs like Bio 1 and 2, Chem 1 and 2, Orgo 1 and 2 and Physics 1and 2 then I would say that they aren't necessarily hard in themselves, but you will need to do alot of work in order to do well, especially if you don't have a science background. If you are from a non sci background its basically all about memorization and connecting different concepts, this takes time. Chem and Physics are more math based so repetition is needed in order to become comfortable with the formulas and ideas and how they are applied over many different practice questions.
So in general its not so much the subject matter, but the amount of time it will take to cover it all, if you are reasonably smart, committed and have good study habits you should be fine.