NEW YORK (AP) -- A new study suggests that teenagers who routinely smoke marijuana may end up with a long-term drop in their IQ.

The researchers didn't find that trend in people who became frequent users after 18. That implies marijuana is especially harmful to the developing brain, an idea that fits in with earlier research.

The study looked at how IQ scores changed between ages 13 and 38 in participants from New Zealand. Experts said the findings aren't definitive, but that the study's methods get around criticisms aimed at some earlier research.

Pot smoking is relatively common in American teens. The government reported in June that 23 percent of high school students said they'd recently smoked marijuana, making it more popular than cigarettes.