"I'm already a happy person, but I smoke pot to have more fun," says Jason, 16, of Pasadena, CA.

Like Jason, nearly 50 percent of American teens have used marijuana at least once in their lifetime. Almost 10 percent of teens have used cocaine, and eight percent have used Ecstasy, according to the 1999 Youth Risk Behavior Surveillance System.

But then there's Daryl, a 15-year-old from New Jersey. He refuses to even try drugs.

"My dad did a lot of drugs, and I can see that they're not the best way to have a good time," he says.

Whether or not teens talk about it openly, drugs are a big issue in their lives. But what makes a teen (like Jason) say "yes" to drugs and another (like Daryl) say "no"?

Reasons Why

"There are several main reasons teens decide to use drugs, and for some teens, it's peer selection, not peer pressure," says Alvera Stern, acting director of prevention application and education at the U.S. Center for Substance Abuse Prevention, in Rockville, MD.

"Peer selection" means that teens start using because the friends they've selected use drugs, not because their friends pressure them, according to Stern.

Jessica, 16, from Kingman, AZ, is one of these teens. She says that drugs really aren't an "important" part of her life, but almost every weekend she gets high on pot laced with a little cocaine or Ecstasy.

"I do it because the people I hang out with do it," she says.

Like Jessica, many teens choose drugs because of their friends, according to Stern, who explains that some teens hang out with an older crowd and want to feel grown-up.

Stern says that other teens start using drugs by accident, because people sneak them into drinks at parties and they like the feeling they get. Some use drugs to relieve stress and cover up emotional pain, or because they like to take risks or rebel, and they use drugs to do so.

"Teenagers are at that point in life where they can do lots of thing they're not supposed to. And drug use may become associated with adult living, which some youth may want to achieve," adds Steven Sussman, Ph.D., professor of preventive medicine at the University of Southern California, in Los Angeles.

Staying Clean

But for all the teens using drugs, there are many who don't need drugs to relieve stress, have fun, or fit in. Many teens don't use drugs because they have positive self-esteem, and satisfying relationships and activities. They don't need to do drugs to make them feel better, according to Stern.

"They've been raised in families with spoken or unspoken values that let them know they can achieve and live happy lives with positive futures," says Stern.

Take Thomas, 17, of Montclair, NJ, whose personal values led him to abstain from drugs.

"After seeing people who did drugs and had bad health problems from it, and seeing the stupid things they did, doing drugs just didn't seem right. I also have a younger sister and I don't want to ruin impressionable minds," he says.

Some teens go beyond abstaining from drugs by adopting a "straight-edge" lifestyle and avoiding self-damaging behaviors.

"I think drug use is stupid and it makes no sense why anyone would want to do it," says straight-edger Ashley, 16, of Spokane, WA.

But in some cases, teens decide to abstain from drugs only after experiencing the negative effects. They quickly decide that using drugs isn't the wisest way to live life.

A.J., 14, from North Augusta, SC, is one of those teens.

"I started doing drugs to make me feel good and to be with the in crowd. But drugs completely messed up my life. My grades started dropping, and I got in a lot of trouble because of it," says A.J.

"I wouldn't recommend going through the experience at all. It's not worth it."

It's important for teens to be well informed about the dangers of drug use and familiar with positive alternatives. Stern advises teens to get the right information about drugs, find ways to manage stress, and learn to communicate with friends and family when they have a problem.

Sex, Etc. Contributor Lindsay Edrington, 15, of North Augusta, SC, contributed to this story.

Editors' Note: Are you a teen with a drug problem who needs help finding a treatment center? The Center for Substance Abuse Treatment has a toll-free, 24-hour treatment and support group referral service. Call 1-800-662-HELP (4357). Or click here.