WEDNESDAY, June 29 (HealthDay News) -- A new study reveals that 90 percent of Americans who are addicted to tobacco, alcohol or other substances started smoking, drinking or using drugs before they were 18 years old.

The study also found that one-quarter of Americans who began using any addictive substance before age 18 are addicted, compared with one in 25 Americans who started using an addictive substance when they were 21 or older.

And nearly half of American high school students now smoke, drink or use other drugs, according to the researchers at the National Center on Addiction and Substance Abuse (CASA) at Columbia University.

They said their findings show that adolescence is the most important period of life for the start of substance abuse and its consequences.

"Addiction is a disease that in most cases begins in adolescence, so preventing or delaying teens from using alcohol, tobacco or other drugs for as long as possible is crucial to their health and safety," Susan Foster, CASA's vice president and director of policy research and analysis, said in a CASA news release. "We rightfully worry about other teen health problems like obesity, depression or bullying, but we turn a blind eye to a more common and deadly epidemic that we can in fact prevent."

In teenagers, the brain is not fully developed, increasing the chances that they'll take risks, including using addictive substances that hamper brain development, impair judgment and increase the risk of addiction, the study authors explained.

The investigators found that 75 percent (10 million) of all U.S. high school students have used addictive substances such as tobacco, alcohol, marijuana or cocaine, and that 20 percent of these students meet the medical criteria for addiction. Currently, 46 percent (6.1 million) of all U.S. high school students use addictive substances and one-third of them meet the medical criteria for addiction.

Nearly three-quarters (72.5 percent) of U.S. high school students have consumed alcohol, 46.3 percent have smoked cigarettes, almost 37 percent have used marijuana, about 15 percent have misused prescription drugs, and over 65 percent have used more than one addictive substance, the study authors stated.

A number of social factors are linked to the high risk of addictive substance use by American teens, according to the researchers. These include: acceptance of substance use by parents, schools and communities; widespread advertising of products such as alcohol and cigarettes; and media depictions of substance use as harmless, glamorous, fun and relaxing.

In addition, tobacco, alcohol, marijuana and prescription drugs are widely available to teens.

Teen substance use is the largest preventable and most expensive public health problem in the United States, according to the study. The immediate costs per year of teen substance use include an estimated $68 billion associated with underage drinking and $14 billion in substance-related juvenile justice costs.

The total national cost of substance use by people of all ages is at least $468 billion per year, or nearly $1,500 for every person in the nation, the report indicates.

"The combination of adolescence, an American culture that glorifies and promotes substance use, and easy access to tobacco, alcohol and other drugs creates a perfect storm for our teens and for taxpayers," Jim Ramstad, a CASA board member who also chaired the report's National Advisory Commission, said in the news release.

"We no longer can justify writing off adolescent substance use as bad behavior, as a rite of passage or as kids just being kids. The science is too clear, the facts are too compelling, the health and social consequences are too devastating and the costs are simply too high," he added.

The study offers a number of recommendations including:

• Educating the public that teen substance use is a public health problem and that addiction is a medical problem that in most cases begins in adolescence.
• Using effective public health strategies to prevent or delay the start of substance use.
• Routine screenings to identify teens most at risk.
• Early intervention to prevent further substance use and consequences.
• Giving teens with substance use disorders appropriate medical treatment.

The study findings are based on online surveys of 1,000 high school students, 1,000 parents of high school students and 500 school personnel; analyses of seven national data sets; interviews with 50 leading experts; five focus groups with students, parents and school personnel; and a review of 2,000 scientific articles and reports.

More information

The American Academy of Pediatrics offers resources about children and substance abuse.