Bringing any type of drug into or out of a country is illegal in most places, so don’t bring them with you when traveling (or at least make sure it’s legal first).
Driving under influence of drugs is also illegal in most places, and for good reason. Do not drive under influence to avoid any problems.
Many places do not allow the use of drugs in public, even when they are legalized for recreational use. Make sure it is legal before lighting a joint in a public place.
Massachusetts decriminalized marijuana in 2008, which greatly reduced the penalties for those who broke the law, but only in 2012 it became legal for qualified patients to consume cannabis products for medical use safely, without the need to worry about running into problems with law enforcements. The first dispensaries for legal purchase of medical marijuana in the state became available later in the 2015.
On November 2016, Massachusetts became the leading state on the whole East Coast to fully legalize the consumption of marijuana products for both medical and rereational use, but you still have to light your joints discretely and out of any public places, as is the case in other states.