With medical marijuana now legal in 21 states and the District of Columbia, and with several other legislatures now considering laws that would legalize it, it’s hardly news that cannabis is big business—or that the social stigma associated with smoking it is slowly fading away.

Even so, Workplace Options, a company that designs employee assistance programs (EAPs) and other corporate wellness efforts, got a surprise when its researchers polled employees about their attitudes toward weed. It turns out that workers aged 30 and under are a lot more likely than their older coworkers to see smoking pot as no big deal.

Consider: Almost half (46%) of employees over 30 said they would “lose confidence” in dope-smoking colleagues, even in places like Colorado and Washington, where recreational use is no longer against state law. These folks are “concerned about having to pick up the slack for a coworker who may not be operating at full capacity,” notes Dean Debnam, Workplace Options’ CEO.

Partly because THC, the active ingredient in marijuana, stays in the bloodstream much longer than, say, alcohol does, “employees are wary of the thought that their coworkers can get high after work and then come in the next day and function with no impact on their performance,” Debnam says.

Employees under age 30, however, have far fewer qualms: 70% of Millennials in the survey would not hesitate to rely on a colleague who smoked weed outside the workplace—maybe because they’re more likely to have done so themselves. More than one in four (26%) told the pollsters they had smoked pot within 12 hours of starting their workday, versus just 7% of the over-30 crowd who said they had.

That “stark difference” in attitudes should give employers pause, Debnam says. For one thing, new college grads and other young hires may not realize that, even in states where marijuana has been legalized, people can still be fired for smoking it.

Right now, many companies have zero-tolerance drug policies, both for safety reasons—in workplaces where people operate heavy machinery, for instance—and in other businesses because “you don’t want people who are stoned talking to your clients,” Debnam says. But “it may be time for employers to review their drug policies” and make sure the rules and their consequences are “crystal clear,” he adds—especially to those employees (and interns) born in 1984 or after.