An estimated 68 percent of Massachusetts marijuana sales this year have taken place outside of the state-regulated market, according to an analysis done for CommonWealth by cannabis market research firm BDSA. The inference is that it is cheaper and easier to buy marijuana on the black market.

A strong black market creates all sorts of challenges. Legalization sent a signal that marijuana is safe to use, but the safety that comes with regulation is lost if consumers are buying the product on the black market instead. The continuing existence of the illegal market also means the potential ties between problem usage and legalization are harder to identify and more difficult to control.

“We’ve done the worst of all possible things,” said John Scheft, attorney who runs an Arlington-based police training company and who opposed marijuana legalization. “We have a legalized system, but we’ve done nothing with black market sales.”