“Teenagers are going to do what they’re going to do,” a friend told me last week when I asked her if her 15- and 17-year-old sons used alcohol or drugs.

“I try to walk that fine line between being disapproving yet open,” my friend said. “I’d rather know than not know what they’re doing. But honestly I don’t think what I say makes that much of a difference. I keep reminding myself that I drank, smoked pot, and tried coke a few times and I’m okay.”

Her explanations sounded reasonable enough to me last week when I began to think about the subject of teenagers and experimentation. I asked dozens of acquaintances how they handled their children’s interaction with alcohol and drugs. Several parents echoed my friend’s acceptance of the reality that teenagers more often than not try these substances during high school.

“More than anything, I want her to be able to talk to me,” one father said. “I don’t want her to be scared to call us if she’s at some party and needs us at one in morning. We’re praying that she makes it through to the other side of these years.”

There is more than prayer involved. There are specific actions parents can take to reduce the chances of their teenagers engaging in dangerous behavior. Contrary to what parents might think, what they say and do makes a difference.

In fact, according to the National Center on Addiction and Substance Abuse at Columbia University, parents are the most important guides for preventing teenagers from abusing substances.

That is, of course, when parents realize that their job is to be parents, not friends.

“Most parents that I see spend far too much time talking about wanting to have an open relationship with their teenager, than thinking about how to effectively communicate rules and boundaries and the reality that some behaviors are unacceptable,” an Upper East Side psychologist said. “As far as I see it, this is a huge problem.”

Another problem cited by an addiction expert is that today’s generation of parents often doesn’t think marijuana is as addictive as it really is. “I treat all these kids and their parents say the same thing over and over again: ‘I smoked a lot of pot and it wasn’t such a big deal,'” the expert said.A 2004 CASA study found that emergency room mentions of marijuana among 12- to 17-year-old patients jumped 48% since 1999.

The first practical step parents can take is to eat dinner as a family. According to a CASA study released last month, teenagers who have two or fewer family dinners a week are twice as likely to smoke daily, try marijuana, and get drunk monthly, compared to teens who have family dinners at least five times a week.

Second, if your teenager is going to have a party at your house, and you have decided to be present at the party, your job is to do more than just attend by controlling the presence of alcohol and drugs at the party.

Third, parents need to talk to their 11-, 12-, and 13-year-olds about alcohol and drugs. Compared to 13-year-olds, 14-year-olds are three times likelier to be offered marijuana or ecstasy. Ninety-five percent of teens who smoke cigarettes start by the age of 15 — the first cigarette, on average, is at 12. Switch the subject to alcohol and marijuana and the statistics hardly budge. According to the 2004 New York City Community Health Survey, 31% of New York City public high school students had tried alcohol before the age of 13.

Fourthly, stop allowing teenagers to watch R-rated movies. A 2005 CASA survey found that 12- to 17-year-olds who watch three or more R-rated movies a month are nearly seven times likelier to smoke cigarettes, more than five times likelier to drink alcohol, and more than six times likelier to try marijuana than those who do not watch R-rated movies.

Finally, it’s helpful to acknowledge that older siblings play an enormous role in determining a teenager’s behavior. A 2002 CASA survey found that 67% of teens with an older sibling say their older brothers or sisters would be “very angry” if they were using marijuana. These children are at a substantially lower risk of drug abuse than the 48% of teens who think that their older siblings have tried illegal drugs.

We parents can make a difference and need to rise to the occasion. We should set rules and stick to them. Set a good example and surround teenagers with role models. And now, more than ever, we should remind ourselves of that public safety message that airs every night: It’s 10 p.m. Do you know where your children are?