Reuters reports that the findings add weight to previous research which linked psychosis with the drug (particularly in its most potent form as "skunk") and will feed the debate about the level of controls over its use.

Despite laws against it, up to 190 million people around the world use cannabis, according to United Nations estimates, equating to about 4 percent of the adult population.

John McGrath of the Queensland Brain Institute in Australia studied more than 3,801 men and women born between 1981 and 1984 and followed up with them after 21 years to ask about their cannabis use and assess them for psychotic episodes. Around 18 percent reported using cannabis for three or fewer years, 16 percent for four to five years, and 14 percent for six or more years.

"Compared with those who had never used cannabis, young adults who had six or more years since first use of cannabis were twice as likely to develop a non-affective psychosis (such as schizophrenia)," McGrath wrote in a study published in the Archives of General Psychiatry journal.

They were also four times as likely to have high scores in clinical tests of delusion, he wrote, and a so-called "dose-response" relationship showed that the longer the duration since first cannabis use, the higher the risk of psychosis-related symptoms.

A study by British scientists last year suggested that people who smoke “skunk,” a potent form of cannabis, are almost seven times more likely to develop psychotic illnesses such as schizophrenia than those who smoke "hash" or cannabis resin.

Previous studies had also suggested smoking cannabis can double the risk of psychosis, but the British study was the first to look specifically at skunk. Skunk has higher amounts of the psychoactive ingredient THC which can produce psychotic symptoms such as hallucinations, delusions and paranoia.

McGrath said, however, that "the nature of the relationship between psychosis and cannabis use is by no means simple," and that more research was needed to examine the mechanisms at work.

As part of his study, McGrath and his team looked at links between cannabis use and psychotic symptoms among a group of 228 sibling pairs and found the association still held. This suggests that other influences like genes or the environment were less likely to be responsible for the psychosis, they said.

An international group of drug policy experts published a book earlier this year arguing that laws against cannabis have failed to cut its use but instead led to vast numbers of arrests for drug possession in countries like Britain, Switzerland, and the United States, which cause social division and pointless government expense.

Other research has supported the idea that there is actually at times a reverse gateway effect, in which social or individual circumstances are the factors responsible for initiating illicit drug use (Lessem et al., 2006).

A study by Tarter, Vanyukov, Kirisci, Reynolds and Clark published in 2006 reviewed the gateway hypothesis and studied whether it exposed a pattern for the transition from licit to illicit drugs.

The study used 224 male participants aged between 10 and 12 years of age, and assessed them at the ages of 12 to 14, 16, 19 and at age 22. Each participant was the son of men who were participating in another study, which included subjects who had a history of illicit drug use.

During each administration of the questioning, researchers used the Structural Clinical Interview for DSM-III-R to assess whether the participants met criteria for substance abuse disorders. The participants also self-administered the Drug Use Screening Inventory and took drug tests (both urine tests and alcohol breath tests) to obtain evidence of recent activity. In addition to these drug-use related screenings, the researchers also questioned the participants about social, family, psychological and educational characteristics and measured them using 35 variables.

The participants were grouped according to three different sequences of drug and alcohol use. The first group was comprised of participants who used alcohol and/or tobacco at the baseline, but had not moved on to using marijuana by age 22. The second group used alcohol and/or tobacco and then used marijuana by the age of 22. The third group used marijuana and then later used alcohol and/or tobacco by the age of 22.

The researchers found that the participants did not necessarily follow the pattern proposed by the gateway hypothesis. Participants whose behavior supported the gateway hypothesis were also associated with other social and family circumstances such as deviant behavior, a deviant social network, were less involved at school, and more likely to discontinue substance abuse than their counterparts who used only licit drugs up to age 22.

The study has limitations, including that much of the information was obtained using self-report, which is subject to bias. Some participants may exaggerate use to dramatize their situation and others will under-report use because of fear of legal sanctions.

The study is helpful, however, in looking at the gateway hypothesis more clearly. The gateway hypothesis is not applicable in all circumstances, nor are drug behaviors consistent across users of a specific drug. This study suggests that instead the patterns of drug use are influenced by a group of general social risk factors, such as parental influence and involvement in school.
 

Science Daily issued a report examining a current edition of Deutsches Ärzteblatt International. Ulrich S. Zimmerman from the Dresden Technical University and his colleagues describe a young man who developed physical withdrawal symptoms after the regular consumption of Spice Gold. These symptoms were also accompanied by a dependence syndrome.

In Germany, Spice Gold has been subject to the German Narcotics Law which forbids the production, free trade and possession of the drug, but initially only for one year. This stipulation has been in effect since January 22, 2009, but is expected to be a permanent regulation by the end of the year.

The featured subject in the study came to the hospital after consuming Spice Gold daily for eight months. He experienced continuous cravings for the drugs and continued to consume the drug, though it caused cognitive impairment. As a result, the individual neglected his duties at work and was threatened with unemployment.

This individual also faced forced abstinence due to a bottleneck in supplies. This abstinence triggered typical withdrawal symptoms, including internal unrest, tremor, palpitations, headache, nausea, vomiting, depression and desperation. All symptoms abruptly disappeared when Spice Gold was once again consumed. When the drug was withdrawn in the hospital, the patient suffered similar symptoms.

The authors interpret these symptoms as an indication of a typical withdrawal disease, most likely due to the admixture of synthetic cannabinomimetics.