The federal and private loans available through the school, which you do not need to make any payments on until you are done, are limited by your budget. Your budget can only include expenses necessary for you alone to complete your education. This includes tuition, supplies, room, board, car maintenance, and maybe a small personal allotment. If you have a family or are supporting children the only addition you can get put on your budget is for child-care expenses while you are taking classes. Some schools have small variations but it is quite similar across the board as the federal government pretty much restricts what you can use the money for. Also, there is a yearly and lifetime cap for federal loans so if youre in a private school with tuition over 30k you will probably need private loans just to make your budget unless you really tighten your belt and have no outside commitments (i.e. credit cards, car loans, mortgage, child support).
You may use credit cards and outside loans if you wish and have the credit rating, it is your life after-all, but you will probably be responsible for making payments on them immediately and monthly and you will be paying them with loan money which is mostly accumulating interest so it is a very unwise way to spend money.
Also realize that the interest that accumulates on your loans (all but the small portion that is subsidized by the government) will be capitalized into your loan once you finish and you will begin paying interest on the interest. If you make no payments during residency and defer all the way through this may happen multiple times and you will pay interest on the interest on the interest!
Dont get all freaked out because you will make enough money to pay it all back and if you are smart along the way it will be very manageable. However, do not think that a little more here or there is insignificant because it can cause huge waves later. In all, spend wisely and live like a student, and of course consider cost of tuition when making a decision on a school. If a school offers you a scholarship or a school is significantly cheaper that should be a big factor in your decision although some people on here will argue otherwise, think for yourself. Good luck