hyperemesis syndrome. a 16-month survey of oregon and alaska emergency departments found a report of the death of an adult who had been admitted for acute cannabis toxicity. long-term main article: long-term effects of cannabis addiction experts in psychiatry, chemistry, pharmacology, forensic science, epidemiology, and the police and legal services engaged in delphic analysis regarding 20 popular recreational drugs. cannabis was ranked 11th in dependence, 17th in physical harm, and 10th in social harm. psychological effects a 2015 meta-analysis found that, although a longer period of abstinence was associated with smaller magnitudes of impairment, both retrospective and prospective memory were impaired in cannabis users. the authors concluded that some, but not all, of the deficits associated with cannabis use were reversible. a 2012 meta-analysis found that deficits in most domains of cognition persisted beyond the acute period of intoxication, but was not evident in studies where subjects were abstinent for more than 25 days. few high quality studies have been performed on the long-term effects of cannabis on cognition, and the results were generally inconsistent. furthermore, effect sizes of significant findings were generally small. one review concluded that, although most cognitive faculties were unimpaired by cannabis use, residual deficits occurred in executive functions. impairments in executive functioning are most consistently found in older populations, which may reflect heavier cannabis exposure, or developmental effects associated with adolescent cannabis use. one review found three prospective cohort studies that examined the relationship between self-reported cannabis use and intelligence quotient (iq). the study following the largest number of heavy cannabis users reported that iq declined between ages 7–13 and age 38. poorer school performance and increased incidence of leaving school early were both associated with cannabis use, although a causal relationship was not established. cannabis users demonstrated increased activity in task-related brain regions, consistent with reduced processing efficiency. a reduced quality of life is associated with heavy cannabis use, although the relationship is inconsistent and weaker than for tobacco and other substances. the direction of cause and effect, however, is unclear. the long-term effects of cannabis are not clear. there are concerns surrounding memory and cognition problems, risk of addiction, and the risk of schizophrenia in young people. neuroimaging although global abnormalities in white matter and grey matter are not consistently associated with heavy cannabis use, reduced hippocampal volume is consistently found. amygdala abnormalities are sometimes reported, although findings are inconsistent. cannabis use is associated with increased recruitment of task-related areas, such as the dorsolateral prefrontal cortex, which is thought to reflect compensatory activity due to reduced processing efficiency. cannabis use is also associated with downregulation of cb1 receptors. the magnitude of down regulation is associated with cumulative cannabis exposure, and is reversed after one month of abstinence. there is limited evidence that chronic cannabis use can reduce levels of glutamate metabolites in the human brain. cannabis dependence main article: cannabis dependence about 9% of those who experiment with marijuana eventually become dependent according to dsm-iv (1994) criteria. a 2013 review estimates daily use is associated with a 10–20% rate of dependence. the highest risk of cannabis dependence is found in those with a history of poor academic achievement, deviant behavior in childhood and adolescence, rebelliousness, poor parental relationships, or a parental history of drug and alcohol problems. of daily users, about 50% experience withdrawal upon cessation of use (i.e. are dependent), characterized by sleep problems, irritability, dysphoria, and craving. cannabis withdrawal is less severe than withdrawal from alcohol. according to dsm-v criteria, 9% of those who are exposed to cannabis develop cannabis use disorder, compared to 20% for cocaine, 23% for alcohol and 68% for nicotine. cannabis use disorder in the dsm-v involves a combination of dsm-iv criteria for cannabis abuse and dependence, plus the addition of craving, without the criterion related to legal troubles. psychiatric