It appears that habitual cannabis use, particularly from adolescence, can result in permanent reduction in IQ:
An international team found those who started using cannabis below the age of 18 - while their brains were still developing - suffered a drop in IQ.
A UK expert said the research might explain why people who use the drug often seem to under-achieve.
This might explain why
Cheech and Chong's Potheads Make Better Quantum Physicists never got beyond the I-just-made-it-up stage.