I might be in the minority here, but I think that the OP has a serious concern. Regardless of what your opinion about pot is, it is illegal and its possession is subject to stiff penalties in many states. If the lease is in your name, tell your roomate that he cannot keep his pot in your apartment--period. The last thing you need is a possession charge.
If you are found guilty of a drug charge, your ability to secure federal financial aid will be severely compromised, which means that you may not be able to complete medical school. Moreover, drug charges may preclude you from applying for a medical license, even if you graduate after losing your student loans.
If you allow your room mate to keep pot in your apartment, you are exposing yourself to these risks. Sure, if he's caught, you might try to tell the cops that the pot isn't yours, but do you want to risk the chance that they might not believe you? What if you get the one officer gunning for convictions? If you're not even enjoying the pot, why take that risk at all? It's awfully selfish of your roomate to jeopardize your future, just so he can have his fix.
P.S. It is absolutely impossible to test positive on a drug screen due to secondhand smoke exposure. This has been proven! Still, allowing your roomate to smoke in your apartment is a bad idea.
Don't do the crime, if you can't do the time. It just isn't worth the risk, however small.
