It affects different people differently. Medicine is obviously an insanely difficult course but I do know folks who manage to get lots of great grades aswell as maintaining a weekend/part-time job.
You can also work out exchanges with your workplace, tell them you'll work extra hours during the summer/holidays and in return ask if they'll give you much less hours as exam time approaches. That way you'll minimise any adverse affects that working has on your uni performence.
If not, then you can just work full time during the summers to fund the rest of your year! And remember, a lot of medical courses do paid work placements as part of the course...