[planetdoc]

not a very good study.

Breiter and his team analyzed a very small sample of patients between the ages of 18 and 25: 20 marijuana users and 20 well-matched control subjects.

small sample size which limits the power of the study.

“We need to see what happens longitudinally,” Breiter said. “What happens as you follow people over time? What happens if they stop using – do these bad effects continue? What happens if you can intervene early?...My worry is we haven’t studied this compound and here we are looking to change legislation on it.”

Although Breiter’s team members did not examine the patients’ cognitive symptoms, they do believe that the brain abnormalities seen in their study could lead to substantial effects on brain development and behavior, especially given the young ages of the participants. Breiter also acknowledged the problems of analyzing a very small study sample – but said that their findings should still serve as a wake-up call to others.

Here is some info from larger studies. His questions are disingenous as a review of literature would answer his questions.

Long-term outcomes — Adverse consequences of cannabis use may diminish or disappear with sustained abstinence or reductions in use. As an example, a study found that previous heavy marijuana use was not associated with adverse effects on physical and mental health, quality of life, and sociodemographic measures [31]. These findings came from a study of 56 monozygotic male twins discordant for cannabis use, in which the cannabis using twins consumed cannabis for a mean of 1085 days, and had last used cannabis a mean of 20 years earlier. The control co-twins used cannabis for a maximum of five days in their lifetime. The twins reported no other significant illicit drug use. The study design controlled for many confounders because the twins had the same genes and shared many of the same environmental experiences during childhood. There were no significant differences between the former marijuana using twins and their siblings for:

Current level of education and household income
Proportion who were married and employed
Current level of health related quality of life
Lifetime nicotine abuse or dependence
Lifetime alcohol abuse or dependence
Lifetime risk of various mood or anxiety disorders
Medication use for medical problems in the past five years
Number of outpatient or emergency room visits in the past five years
Number of hospitalizations in the past five years

Eisen SA, Chantarujikapong S, Xian H, et al. Does marijuana use have residual adverse effects on self-reported health measures, socio-demographics and quality of life? A monozygotic co-twin control study in men. Addiction 2002; 97:1137.

Neuropsychological effects — The long-term residual neurocognitive effects of cannabis abuse and dependence appear to be minimal, but some of the evidence is conflicting.

A meta-analysis including more than 600 cannabis users found no evidence that marijuana causes significant long term effects on neurocognitive processes [39]. These results were consistent with an epidemiologic study [40].
A study, completed after the meta analysis, administered a comprehensive neuropsychological test battery to monozygotic twin pairs discordant for regular marijuana use, in which neither twin used any other illicit substance regularly [41]. Testing was completed a minimum of one year after marijuana had last been used, with a mean of almost 20 years since last regular use. A total of 56 tests assessed the domains of general intelligence, attention, memory, executive functioning, and motor skills. Marijuana users performed significantly worse on only one of the tests compared with those who did not use marijuana.

While other studies have found evidence of deficits in attention, memory, executive functioning, and psychomotor speed related to dose and cumulative exposure to cannabis, these studies have involved smaller patient groups as well as potential confounding by ongoing or recent cannabis use and other factors [42-44].

Neuroimaging studies have attempted to correlate structural and neurophysiologic changes with long-term cannabis use. In one study, magnetic resonance imaging showed that long term, heavy use is associated with significantly reduced volumes in the hippocampus and amygdala (two regions of the brain rich in cannabinoid receptors) [45]. In addition, a greater cumulative cannabis exposure is associated with more severe volume reductions. Other investigations using positron emission tomography, transcranial Doppler sonography, and EEG, have all suggested neurophysiologic changes in chronic cannabis users [46-48].

from uptodate. I've posted it on this forum before....so please search for it. I dont want to muddy this post with all the citations that do exist.

Drug use of any kind is not recommended on someone with a developing brain (unless the benefits outweigh the risks). There is really no data showing that cannabis has any permanent neurological effects on a fully developed brain.