Without seeing your particular lease agreement, I would recommend sending your complaint about the smoking to the landlord via certified US mail (and keep a copy). There is a good chance your landlord's inaction in allowing your next-door neighbor to smoke breaches the 'warranty of habitability.' Put your complaint in writing, then I would recommend having an attorney review your Lease to see when you could terminate the agreement.
This information is given for legal education only. It may not work for your specific situation. It is not legal advice, and I am not your lawyer. You have to find your own local lawyer to get legal advice and help with your problem.