Teens who use pot could pay for it later in life.

A decades-long study of more than 1,000 New Zealanders suggests teenagers who routinely smoke marijuana risk a long-term drop in IQ.

Researchers said this finding fits with other signs that drug use is especially harmful to the developing brain. Participants over the age of 18 did not have the same IQ dip.

"Parents should understand that their adolescents are particularly vulnerable," lead researcher Madeline Meier, with Duke University, said.

"IQ is a strong determinate of a person's access to college education, getting a job, performance on the job, tendency to develop heart disease," she explained. "So those individuals who lose IQ points may be disadvantaged toward the most important aspects of life."

Pot is the most popular illegal drug worldwide. Twenty-three percent of high school students in the United States said they had recently smoked marijuana.