By David Crary, AP

NEW YORK — Experimentation with human growth hormones by America’s teens more than doubled in the past year, as more young people looked to drugs to boost their athletic performance and improve their looks, according to a new, large-scale national survey.

In a confidential 2013 survey of 3,705 high school students, released Wednesday by the Partnership for Drug-Free Kids, 11 percent reported using synthetic HGH at least once — up from about 5 percent in the four preceding annual surveys. Teen use of steroids increased from 5 percent to 7 percent over the same period, the survey found.

Travis Tygart, CEO of the U.S. Anti-Doping Agency, depicted the numbers as alarming but not surprising, given the extensive online marketing of performance-enhancing substances and near-total lack of any drug testing for high school athletes.

“It’s what you get when you combine aggressive promotion from for-profit companies with a vulnerable target — kids who want a quick fix and don’t care about health risk,” Tygart said in an interview. “It’s a very easy sell, unfortunately.”

Nine percent of teen girls reported trying synthetic HGH and 12 percent of boys.

“A picture emerges of teens — both boys and girls — entering a largely unregulated marketplace (online and in-store) in which performance-enhancing substances of many varieties are aggressively promoted with promises of improved muscle mass, performance and appearance,” said the report. “This is an area of apparently growing interest and potential danger to teens that cries out for stricter controls on manufacture and marketing.”

Given the high cost of authentic HGH, it’s possible that some of teens who reported using it may in fact have obtained fake products. As the survey said, “It’s very difficult to know what exactly is in the substances teens are consuming, or what the short- and long-term impact on their health may be.”

Steve Pasierb, president of the Partnership for Drug-Free Kids, said the motives of today’s youthful dopers were different from the rebellious or escapist attitudes that traditionally accompanied teen drinking and pot-smoking.

“This is about how you feel, how you look,” Pasierb said. “They’re doing this thing to get ahead. … Girls want to be thin and toned. For a lot of boys, it’s about their six-pack.”

He urged parents to talk candidly with their children about the dangers of performance-enhancing substances, but to avoid moralizing.

“It’s not about illegality, or whether you’re a good parent or bad parent,” he said. “It’s a health issue. These substances literally alter your body.”

Pasierb said high school coaches have a key role in combating doping. Some are vigilant, other oblivious and perhaps a third are prepared to tolerate doping in the interests of winning, he said.

The new survey noted that the upsurge in teen HGH use occurred even as famous athletes were caught up in high-profile doping cases. Last August, Major League Baseball punished Alex Rodriguez with a lengthy suspension after investigating his use of performance-enhancing drugs. A few months earlier, Lance Armstrong admitted in a TV interview to doping throughout his cycling career.

One of Armstrong’s former teammates is Tyler Hamilton, who was forced to return his 2004 Olympic gold medal after being found guilty of doping. In recent public appearances, Hamilton has implored young athletes to resist the temptation to dope.

“There’s so much pressure on winning — it’s tough for these kids to stay true to themselves,” he said. “I can’t change every kid’s mind, but if I can do my part and other people do their part, we can beat this monster.”

Tygart, who as USADA’s chief oversaw investigations of Armstrong and Hamilton, noted that stringent testing regimens are an increasingly effective deterrent to doping among athletes in major pro sports and in international competitions.

“But most young athletes are not in any testing program, and their chance of getting caught is zero,” he said. “When left unchecked, the win-at-all-cost culture will take over and athletes will make the wrong decision.”

Synthetic HGH is supposed to be available only by prescription, yet products claiming to contain HGH are widely promoted and enforcement of the regulations is inconsistent, Tygart said.

Among the groups seeking to reverse the teen doping trend is the Texas-based Taylor Hooton Foundation, named after a 17-year-old high school athlete whose suicide in 2003 was blamed by his family on his use of anabolic steroids. Its staff has spoken to thousands of young people at school assemblies and sports camps.

Donald Hooton Sr., Taylor’s father and the foundation’s president, depicted teen doping as an epidemic fueled by widespread ignorance among parents and coaches. He estimated that more than 1.5 million youths in the U.S. have tried steroids.

Information about teen use of performance-enhancing drugs is readily available online. The Mayo Clinic, for example, provides a list of possible hazards and side-effects, including stunted growth, acne, liver problems, shrunken testicles for boys and excess facial hair for girls.

The clinic urges parents to check the ingredients of over-the-counter products used by their teens, and to be on the lookout for warning signs, including increased aggressiveness, rapid weight gain, and needle marks in the buttocks or thighs.

The Partnership for Drug-Free Kids survey also reported on other forms of substance abuse. Among its findings:

—Forty-four percent of teens report using marijuana at least once within their lifetime; 24 percent report using within the past month; and 7 percent report using at least 20 times within the past month. These levels have remained stable over the past five years.

—After a sharp increase in teen misuse and abuse of prescription drugs in 2012, the rate remained stable in 2013, with 23 percent of teens reporting such abuse or misuse at least once. Fifteen percent reported having used the prescription painkillers Vicodin or OxyContin without a prescription at some point.

The survey of 3,705 students in grades 9-12 was conducted at their schools between February and June 2013.

The margin of error was calculated at plus or minus 2.1 percentage points.

Founded in 1987, the New York-based Partnership for Drug-Free Kids is a nonprofit working to reduce teen substance abuse and support families affected by addiction.