It depends on which school you attend, and how naturally adept you are for that school's preferred mode of learning.
In my experience though, the work isn't what kills you - it's the pace. Everyone in Med is a gifted individual, and thus the expectations placed on you are more exacting than in a "regular" science course. Students are expected to learn more material, in greater detail, within a shorter period of time.
Of course, given the demands placed on doctors...can you think of any other way?