Smoking marijuana will certainly affect perception, but it does not cause permanent brain damage, said researchers from the University of California at San Diego. "The findings were kind of a surprise. One might have expected to see more impairment of higher mental function," said Dr. Igor Grant, a UCSD professor of psychiatry and the study's lead author. Other illegal drugs, or even alcohol, can cause brain damage.

His team analyzed data from 15 previously published, controlled studies into the impact of long-term, recreational cannabis use on the neurocognitive ability of adults. The studies tested the mental functions of routine pot smokers, but not while they were actually high.
No effect at all was seen on reaction time, attention, language, reasoning ability, and perceptual and motor skills.

Anecdotal evidence has shown that marijuana can help ease pain in patients with diseases like multiple sclerosis or prevent severe nausea in cancer patients, but the effects have yet to be proven in controlled studies.

Grant noted that heavy marijuana users often abuse other drugs, such as alcohol and amphetamines, which also might have long-term neurological effects.