Teens who live in areas where parents are more approving of pot tend to have a higher rate of pot use, a new study indicates.

Researchers compared data on Oregon marijuana growers and medical marijuana patients with responses to statewide Healthy Teens surveys.

They found that Oregon counties with the highest rates of marijuana growers per capita from 2006 through 2015 also have the highest number of patients per capita during that period.

And many of those same counties had the highest rates of teen marijuana use and the lowest rates of parental disapproval from 2006 through 2015.

"We think our findings suggest that for every percent increase in patients or growers, there is a comparable percent increase in use among kids," said Mallie Paschall, the study's lead author and a scientist at the Prevention Research Center, a nonprofit in Oakland, California.

Josephine County landed on the top of the grower and patients lists, followed by Jackson and Curry counties. Multnomah, Washington and Clackamas counties were in the bottom half of both lists.

The study's general findings didn't surprise Oregon specialists, who said the more marijuana grown and the more people who use it, the more likely it is to be accepted and used by young people.

The same has been true with alcohol and tobacco, said Dr. Paul Lewis, health officer for Clackamas, Multnomah and Washington counties.

Like those two substances, marijuana can also bring problems.

"There are really obvious short-term risks," Lewis said, "like having a bad trip, having trouble driving and getting into car accidents."

The long-term implications worry him the most. Unrelated studies have shown that the younger people are when they start using a substance, the more likely they are to become dependent on it.

Marijuana is "not terribly addictive," said Tony Biglan, co-author of the study and senior scientist at the Oregon Research Institute, a nonprofit in Eugene. "But adolescents who start using it have trouble stopping in adulthood."

Marijuana use can also affect brain development, various studies have shown.

"Individuals under 25 who use on a regular basis will have a fairly significant impact on their cognitive function," said Dr. Stuart Gitlow, an addiction psychiatrist in Rhode Island and past president of the American Society of Addiction Medicine.

He said daily use by young people can lower their intelligence quotient by 8 points. He said that's not enough to affect those who start out with an IQ over 100 but could hamper the rest in the workplace.

The study, published in the Journal of Primary Prevention, was funded by a grant from the National Institute on Alcohol Abuse and Alcoholism. Researchers analyzed county data on growers and patients. It compared that with responses from the Oregon Healthy Teens Survey, which anonymously polls eighth- and 11th graders on substance use and other issues. Gilliam, Sherman, Wallowa and Wheeler counties were excluded because they did not have survey data.

Teens were asked about their marijuana use in the past 30 days and parental disapproval.

In Oregon, voters approved medical marijuana in 1998. The first retail sales of recreational marijuana began in October 2015.

That's not far enough in the past to give researchers a good idea of the effect of recreational sales on young people, Biglan said.

"There is considerable concern that legalization will be associated with an increase in youth marijuana use," Biglan said.

Statewide, 8 percent of eighth-graders and 22 percent of 11th-graders reported current marijuana use, according to an Oregon marijuana report published in December. That's comparable to nationwide teen use, the report said.

Twenty-nine states have legalized medical marijuana and eight states have approved recreational use. Washington, D.C., allows both.

-- Lynne Terry