Ronald Reagan cast a long shadow over American politics. But his wife Nancy has enjoyed her own enduring legacy with the “Just Say No” campaign, which enlisted media — especially geared toward youth — as antidrug crusaders.

A generation later, however, the times they are a-changing. Yes, there are still plenty of shows striking a scolding posture toward drug use. But many others — especially on cable — view hitting a joint or bong as just another aspect of characters’ lives.

For the last few years, cable series like “Weeds,” “Californication,” “Wilfred” and almost anything on Comedy Central have adopted this approach. It was noteworthy, though, to see a recent episode of CBS’ “Two and a Half Men” where the teenage son finds a lost water pipe in his late uncle’s piano and nobody stops to lecture him about the dangers of getting high.

Still, a more notable breakthrough might be coming to reality TV, where drugs have largely been presented in a cautionary mode, from “Intervention” to “Celebrity Rehab With Dr. Drew.” Like most unscripted fare, the tendency has been to dwell on abuse taken to perilous extremes.

Against that backdrop, it’s worth taking note of “Weed Wars,” a Discovery Channel series premiering in early December that focuses on a cannabis dispensary in Oakland, Calif.

While the center boasts an eccentric group of characters, they seem a relatively harmless lot, and speak freely about casually “medicating.” Moreover, one guy explains he began professionally growing pot plants after working as a mortgage banker, which made him feel a whole lot dirtier.

Such breakthroughs are incremental, to be sure, but reflect a gradual evolution, as evidenced by a new Gallup Poll that for the first time found half of respondents support making marijuana legal.

According to Ethan Nadelmann, exec director of the Drug Policy Alliance, dramatic depictions of marijuana are increasingly incidental to the plot, not the thrust of it. “It’s normalization,” he says. “It’s not Cheech & Chong, and it’s not ‘Reefer Madness.’ It’s background.”

Such an environment symbolizes a marked departure from a period as recent as the Clinton administration, when networks eagerly enlisted in the government’s war against drugs.

In the late 1990s, ABC launched a campaign in conjunction with Partnership for a Drug-Free America called March Against Drugs, which included weaving antidrug messages into the network’s daytime and primetime programs.

As Salon noted in 2000, under then-White House drug czar Barry R. McCaffrey, the government even weaseled its way into programming decisions — viewing scripts of shows containing “Just Say No” pitches, under a complex scheme allowing broadcasters to leverage such episodes to reclaim ad time that would otherwise have to go toward antidrug spots.

Admittedly, drugs remain a dicey proposition in TV and movies. The Motion Picture Assn. of America’s rating guidelines state: “Any drug use will initially require at least a PG-13 rating.” Two years ago, the romantic comedy “It’s Complicated” got slapped with an R because grown-up characters played by Meryl Streep and Steve Martin smoked marijuana without consequences.

Sex and coarse language will always garner more attention, perhaps, but drugs have arguably been more taboo.

In that context, portraying pot smoking as part of the scenery does represent a shift. The reasons are various, including the imperative to reach younger viewers, an explosion in niche programming and aging baby-boomers who might preach abstinence to their kids but just said “yes” themselves.

While most teens experiment with pot, Nadelmann says, “This is the first generation of high school students where a majority of their parents have tried marijuana. … There’s a less-hysterical atmosphere.”

In his PBS documentary “Prohibition,” filmmaker Ken Burns titled one chapter “A Nation of Scofflaws.” And millions of Americans have casually scoffed at laws against marijuana for decades.

As Nadelmann concedes, Hollywood remains wary in regard to drugs, though some are more adventurous than others. Snoop Dogg even blessed “Weed Wars” by recording the show’s original title track.

Whatever inroads have been made, though, TV isn’t in the lead on the issue. In its eagerness to find a hip young crowd, it’s simply following the smoke signals. n