Laws regarding possession, sale, and use of marijuana are changing quickly here and Massachusetts and all over the country, and often differ greatly from state to state. These resources will help you understand some of the varying laws.
Massachusetts law
Summaries
Text of Laws
Article
Laws Across the U.S.
Mother Jones - Article with map and table - shows states that allow for reduced penalties, medicinal marijuana and recreational marijuana.
State Medical Marijuana Laws - National Conference on State Legislatures, 11/13/2014
Issues
After 5 Months of Sales, Colorado Sees the Downside of a Legal High - NY Times, 5/31/14