The problem you are facing is that smoke is smoke and knows no real boundaries. The fact that the issue involves your neighbors "use" of medical marijuana does not overcome the fact that their conduct is interfering with your lawful and appropriate use of your rental unit. This is a violation of what is known as the implied covenant of "quiet enjoyment" that is part of residential rental agreements.
My strongest recommendation is that you give notice and vacate the unit even if this means staying some where else until you actually surrender the keys. You may also want to negotiate an earlier move-out date in exchange for surrendering your security deposit.
Once you have moved out, you may then consider suing the landlord for breaching the implied covenant of quiet enjoyment by failing to take steps to protect you from the neighbors' smoking or rather their smoke period. It is hard to say if your claim will succeed but the landlord may agree to pay your reasonable moving expenses, refund your security deposit and provide you with a partial rent abatement, particularly if you have medical documentation of your allergy to smoke and your breathing issues.
I also recommend that you consult a personal injury attorney and a housing law attorney for a more complete understanding of your rights and interests in this matter.
This is not a substitute for a consultation with an attorney with a background in housing and personal injury law.