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As THC levels rise, the risk of psychosis, cognitive deficits, and structural brain changes increases



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)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).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 THC:CBD 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.


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