In the United States, the average potency of Cannabis has increased significantly over the past few decades in response to consumer demand and policies in some states that have legalized marijuana for medicinal and recreational purposes. (1) Whereas the delta-9-tetrahydrocannabinol (THC) content of "street" marijuana was <1% in the 1970s and 4% in the 1990s, by 2012, analyses of Cannabis samples seized by law enforcement agencies documented a rise in average THC potency to >12%. (1-3)
Although this increase in potency has been overstated in the media because studies did not control for the effects of changes in sampling methods on freshness, it is estimated that Cannabis potency increased 7-fold from 1970 to 2010. (3) Also, Cannabis preparations such as hashish and hash oil extracts containing THC well above average--from 35% to 90% THC--are now more widely available. In states where marijuana has been legalized, high-potency Cannabis (HPC) in the form of "edibles" (eg, marijuana added to baked goods, candy, or drinks) and hash oil extracts (Table 1, page 16) (4-13) can be readily obtained from dispensaries or even at local farmers' markets.
The potency of Cannabis, typically defined as the percentage of THC, its chief psychoactive component, varies depending on the genetic strain of the plant, cultivation techniques, and methods of processing and storage. For example, relative to "average marijuana," hemp (Cannabis bred for industrial purposes) has very little THC, while sinsemilla (flowering buds from unpollinated female plants), hashish (Cannabis resin), and extracted hash oil contain increasing amounts of THC (Table 2, page IS). (1,2)
As THC levels in Cannabis have risen over time, cannabidiol (CBD) levels have dropped to <0.2%. (2) Although THC appears to be largely responsible for the psychiatric morbidity associated with Cannabis, CBD may have neuroprotective and antipsychotic properties. (14,15) The sharp spike in the THGCBD ratio in recent years therefore raises the possibility that Cannabis use today might carry a much greater risk of psychiatric sequelae than it did in previous generations.
This article reviews the evidence for an increased risk of psychiatric morbidity with increasing Cannabis potency.
Cannabis use disorder
Recent data indicate that the prevalence of Cannabis use disorders (eg, abuse and dependence) in the United States is approximately 3% among the general population and >30% among Cannabis users. (16) The availability of increasingly potent forms of Cannabis has been cited as a possible explanation for this rise, despite no change in the prevalence of overall marijuana use between 1991 to 1992 and 2001 to 2002. (17) However, while the prevalence of marijuana use disorders has continued to rise--nearly doubling from 2001 to 2002 to 2012 to 2013--this latest increase occurred with a significant increase in overall marijuana use, such that the actual rate of Cannabis use disorders among users seems to have plateaued, despite the continued rise in marijuana potency. (16)
This discrepancy could be explained if Cannabis users cut back past a specific threshold of increasing potency. However, 2 studies have called into question how effective such titration efforts might...
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