Teenagers lighting joints may end up less bright, according to new research  released Monday.

In a study of more than 1,000 adolescents in New Zealand, those who began habitually smoking marijuana before age 18 showed an eight-point drop in IQ between the ages of 13 and 38, a considerable decline.

The average IQ is 100 points. A drop of eight points represents a fall from the 50th percentile to the 29th percentile in terms of intelligence.