Short answer: Yes, you can get out. However, this will be harder than you may want it to be.
You will need to check your lease agreement for an arbitration clause. If the lease mentions disagreements will be handled by arbitration (or an arbiter), you need to know that going into this. Arbitration clauses usually stipulate that the landlord picks the arbiter, who will almost always be predisposed to side with the landlord.
This pamphlet sheds insight into the situation. Skip down to PDF page 22, section header "My neighbors are constantly playing loud music..." Summarized, you can get out but if other tenants are not as affected, it will be hard to prove it affects you differently. You do, however, have the benefit of knowing the particular situation is illegal (as compared to the pamphlet's example of playing music, which is only situationally illegal).
First, get written, signed testimony from other tenants (your roommate, the one above the smoker, etc). Preferably, get these signatures notarized. The testimony should include an acknowledgement that the signer has smelled the marijuana and a statement about how often this occurs, along with the date of signature.
Next, send a certified letter to your landlord. Keep a copy of this letter. In it, state your intention to move unless they fix the situation within thirty (30) days. Remind them you already have informed them of this situation. Inform them you have (hopefully notarized) testimony of the marijuana smoking from other tenants, remind them that this is illegal, and that this is affecting your health. You may also warn them that if they do not rectify the situation and you leave after thirty days, they cannot keep your deposit or charge you a termination fee- they have breached your lease contract through failure to maintain a safe and livable rental unit (a gentle reminder that you will seek legal action if they attempt to keep your deposit or charge you fees is appropriate). In the letter, request an immediate, written response with their intentions on the matter, and give a deadline (like 3 days from receipt). As a certified letter, you will know when they get it.
The wording of this letter is important. You want to clearly state the facts without sounding self-righteous, angry, or vindictive. And proofread the heck out of it.
If the landlord is faced with possible legal action for forcing an illegal tenant to follow the law, and certain legal action for trying to keep a legal tenant to remain in illegal, harmful circumstances, they may well decide to oust the smoker. If your scare tactic doesn't work, however, you should make good on the threat- really do leave after thirty days, and if they attempt to charge you for anything or withhold your deposit, you really should contact a lawyer.
You should also, however, have the money saved to pay the termination fees, just in case. Because the smoke affects you differently, and there is no easy way to prove this, an unsympathetic court (or arbiter) may rule against you.
(NOTE: I am not a lawyer, but I did have to break a lease and spent a good deal of time researching and discussing my situation with a lawyer. In the end I lost my deposit but did not have to pay any fees or missed rent.)