For years, since the first medical marijuana laws were passed in the mid-1990s, many local and state governments could be confident, if not complacent, knowing that marijuana would be kept in check because it remained illegal under federal law, and that hard-nosed federal prosecutors were not about to forget it.

But with the Justice Department’s announcement last week that it would not prosecute people who use marijuana for medical purposes in states where it is legal, local and state officials say they will now have to take on the job themselves…

At least five states, including New York and New Jersey, are considering laws to allow medical marijuana through legislation or voter referendums, in addition to the 13 states where such laws already exist. Even while that is happening, scores of local governments in California, Colorado and other states have gone the other way and imposed bans or moratoriums on distribution even though state law allows it.

Some health and legal experts say the Justice Department’s decision will promote the spread of marijuana for medical uses because local and state officials often take leadership cues from federal policy. That, the experts said, could lead to more liberal rules in states that already have medical marijuana and to more voters and legislators in other states becoming comfortable with the idea of allowing it.