Thanks for your question. I am of the belief that if the landlord has not taken steps to restrain the tenants from engaging in illegal drug use, you may validly terminate your lease on the grounds that the landord has interfered with your right of quiet enjoyment of the apartment. However, in order to minimize the risk associated with this (that risk being a lawsuit by the landlord for rent), I would recommend that you have several disinterested witnesses corroborate your observations about the pot smoke. These witnesses should be people who come across as being extremely credible, and who should anticipate possibly having to testify at a court proceeding. If you would like to discuss this further, please be in touch.
It is certainly possible to break your lease if you wish to do so. But when you breach the terms of your lease agreement, you expose yourself to liability. For example, you may be liable to your landlord for the remainder of your lease term. You may also have certain defenses and counterclaims as a tenant. You would probably benefit from legal counsel in successfully navigating this situation and limiting your exposure. As a start, your lease agreement should be reviewed carefully.