There are currently about 40 million cigarette smokers in the U.S., according to the Centers for Disease Control and Prevention. Given that cigarette use is in decline, marijuana use could become more prevalent than cigarette use in just a few years' time.
There are likely several factors driving these numbers. Since 2013, recreational marijuana markets opened in Colorado and Washington, and several other states voted to legalize marijuana in the fall 2014. It's likely that adults in those places are taking advantage of the new opportunities to indulge legally.
Recreational marijuana use remains illegal at the federal level and in most states. Police are arresting people for possessing marijuana at record-high rates -- more than 1,700 per day, according to 2014 data from the FBI.
Still, attitudes toward marijuana use have come a long way from the "this is your brain on drugs" era of the 1980s and '90s, when Ronald Reagan was calling marijuana "the most dangerous drug in the United States" and top law enforcement officials were publicly calling for marijuana smokers to be "taken out and shot."
Much of this shift in attitudes could be due to lived experience. In the late 1960s, fewer than 5 percent of adults told Gallup they had ever smoked marijuana. Today that number is up to 43 percent. Regardless of whether they use it currently, nearly half of American adults now have first-hand experience using marijuana.