If you have completed a bachelor's (or a master's, or even a doctoral) degree -- but you didn't take the pre-med classes during undergrad and now want to be a physician -- you can take those classes through a "post-bac" (short for post-baccalaureate) pre-medical program.
Simply put, this is a barrage of the pre-med science classes, which gives you the prerequisites and preps you for the MCAT. You have to take these classes, and you have to take the MCAT, in order to be considered for matriculation in a US/Canadian medical school.
Some schools offer official programs, complete with a certificate upon completion and help in the application process to med school. You can also create your own program by taking the sciences yourself, which is what I did.
I know that many, many schools offer official post-bac programs -- in the Boston area, you can find them at Tufts University, Boston University Metropolitan College, and Harvard Extension School. Seriously, though, there are a ton of them out there.
The classes you need, by the way, are a year of biology, a year of chem, a year of organic chem, and a year of physics (all with lab). There are some schools which require biochem, as well.
Good luck, I hope that helps.
