DENVER — Starting early next year, any adult with a craving or curiosity will be able to stroll into a strip mall or downtown shop in Colorado or Washington State and do what has long been forbidden: buy a zip-lock bag of legal marijuana.

After landmark votes made marijuana legal for recreational consumption, users in these two states will no longer need doctors’ notes or medical reasons to buy the drug. Instead, they will simply show identification to prove they are at least 21, and with the cautious blessing of state and federal officials, they will be able to buy as much as an ounce of marijuana and smoke it in their living rooms.

It is a new frontier of drug legalization, one that marks a stark turn away from the eras of “Reefer Madness,” zero tolerance and Just Say No warnings about the dangers of marijuana. But it also raises questions about whether these pioneering states will be able to regulate and contain a drug that is still outlawed across most of the country — although medical marijuana can be sold legally in 20 states and the District of Columbia. The end of the prohibition of alcohol in the 1930s, by contrast, to which some historians and legal scholars are comparing this moment, came all at once across the nation.

On this never-traveled road, the outcome on many fronts is uncertain: Supporters predict an economic boom in new business activity, cannabis tourism and reduced public expense with fewer low-level drug offenders clogging jails and courtrooms.