These analyses confirm global trends towards the dominance of THC content in contemporary cannabis, with these Australian data showing average values similar, if not slightly higher, than recent international studies (Table 1). While there was wide variation in cannabinoid levels, high mean and median values of THC_tot and low values of CBD_tot and other potentially therapeutic cannabinoids are similar to those reported internationally in samples of cannabis identified as sinsemilla, commonly referred to as “skunk” [3], [5], [7].

This pattern of high THC/low CBD cannabis has become a focus of concerns over the potential mental health impacts of current cannabis use patterns. Given existing data on the potential modulating effects of CBD on the adverse effects of THC, these data lend support to the proposition that cannabis currently available in Australia exhibits a profile that may render some cannabis users vulnerable to potential adverse mental health impacts of their use. However, there remains scant research on this issue other than small scale surveys and laboratory studies demonstrating biological plausibility. For example, while there have been noted increases in treatment seeking for cannabis use internationally across the past decade, particularly in young people, there are other conceivable explanations apart from increased potency. These might include improved treatment availability and schemes where users are diverted from the criminal justice system into treatment [33]. Further, while Australian hospital separations for cannabis-induced psychosis increased over the 2000s, particularly among older age groups [28], modelling research does not indicate increases in levels of schizophrenia commensurate with increases in cannabis use [34], [35].

There are also several possible moderators of the impacts of cannabis potency on cannabis users. While there is mixed evidence on use trends, overall cannabis use appears to be stabilising or declining in some regions (e.g., Western Europe, USA and Australia) after increased use throughout the 1990s and early 2000s [8], [26]. Further, effective potency, that is the amount of THC and other relevant cannabinoids actually absorbed by the user, may vary according to such factors as natural variations in the cannabinoid content of plants, the part of the plant consumed (e.g., more potent buds versus leaf material), route of administration (e.g., oral vs. smoking) and user titration of dose to compensate for differing levels of THC in different smoked material [10], [36]. In smoking cannabis, only approximately 30% of THC-A is thought to be converted to free THC [37] with THC, rather than THC-A, providing the main psychoactive effects when cannabis is smoked or vaporized. Thus, THC_tot may not necessarily be an accurate representation of effective potency. On the other hand, the non-psychoactive THC-A content of plants is of increasing interest given its potential medicinal and neuroprotective properties [38]. A recent trend towards “juicing” cannabis plant material for consumption is aimed at maximising THC-A intake, while minimising the intoxicating effects of THC.

Although the overall median levels of CBD were very low in our samples it is interesting to note there were 5 samples that exceeded 1% CBD_tot, with one containing 6.5% CBD_tot. Of these 5 high CBD_tot samples identified, 4 were seized from rural locations and 1 seized from an urban location. CBD-A is also gathering attention for its therapeutic potential, with evidence of anti-emetic [39] and anti-cancer properties [40]. Samples obtained from rural seizures contained higher levels of virtually all measured cannabinoids including trace phytocannabinoids, but most noticeably THC, THC-A and THC_tot. This is not entirely surprising given that regional areas of NSW such as Byron Bay, Lismore and Tweed Heads have long been associated with cannabis use and specialist cultivation approaches. However, it is not currently possible to identify whether urban and rural Cannabis Cautioning seizures are likely to represent cannabis grown using different cultivation methods as the origin of the samples was unknown, and could even reflect cannabis grown hydroponically in urban locations and transported to regional NSW.

There were no differences between known outdoor (Northern NSW) and indoor (Sydney) grown seizures in levels of THC, CBD or other cannabinoids, although there were trends towards higher THC_tot in indoor-grown samples. These data therefore do not provide overwhelming support for claims of higher potency in cannabis grown using intensive indoor cultivation techniques. Given the observed trends towards higher THC, this small sample may have had insufficient power to reliably detect such differences. However, it may simply be that specific types of seed material are favoured for cannabis cultivation, and that this factor dominates cannabinoid profiles rather than the use of outdoor or indoor growing locations. An interesting issue for future research is the value growers place on strains containing high levels of THC and low levels of CBD, as reflected in their preference among many cultivators [8], [41] and higher market prices [6], [8], [42]. Given concerns over the potential mental health impacts of this profile, as well as reports of the aversive nature of the high associated with it by some users [43], research on user preferences associated with different effects might shed light on whether cannabis containing a more balanced mix of THC and CBD would have value in the market, as well as potentially conferring reduced risks to mental wellbeing.

There were relatively high levels of CBG_tot (the precursor molecule to THC-A, CBD-A and CBC-A [32]) when compared to other trace phytocannabinoids, with CBG the second most abundant phytocannabinoid in the seized plant material. Research has found that CBG-A increases up to the twelfth week of cultivation (third week of flowering) and then decreases until the end of cultivation, while CBG increases all the way to the end of cultivation [44]. High CBG in seized cannabis plants may indicate that growers may be allowing their plants to mature before harvesting. As a weak partial agonist at cannabinoid type1 (CB1) and type 2 (CB2) receptors, a highly potent α₂ adrenoceptor agonist, and a moderately potent serotonin-1A (5HT_1A) antagonist [45], there may be a potential use for CBG as an antidepressant and analgesic [46].

We also found trace amounts of the non-psychotropic phytocannabinoid THC-V, which appears to have an antagonistic effect on CB1 receptors, displacing synthetic CB1 agonists CP-55940 and WIN-55212 and attenuating the antinociceptive and hypothermic effects of THC in vivo [47]. However, the THC-V concentrations used to produce an antagonistic response are at least 100–1000 times higher than what would be reasonably absorbed during smoking of a typical joint. CBC, another trace non-psychotropic phytocannabinoid appears to modulate the effect of THC by inhibiting endocannabinoid cellular reuptake, and is also a potent activator of TRPA1 receptors, with apparent analgesic [48] and anti-inflammatory effects [49], [50]. However, like CBD, the trend for maximising THC production may have led to marginalisation of CBC as historically, CBC has sometimes been reported to be the second or third most abundant cannabinoid [51].

Some limitations inherent in the data presented here should be acknowledged. Due to funding constraints we could not collect a very large random or necessarily representative sample of Cannabis Cautioning seizures. However, we did ensure the samples we obtained came from the major rural cannabis growing areas on the NSW north coast and the major urban areas of the state. Further, as both Cannabis Cautioning and Known Provenance samples were not required to be retained for criminal proceedings, we received and stored them soon after they were seized. The freshness of the samples is confirmed by the dominance of carboxylic acid forms of THC, CBD and CBG, and very low levels of CBN, the main oxidation product of THC.

Given the known variability of THC within a single plant [3], it is possible that these results do not represent the “true” average potency of each plant as buds were used whenever possible from samples that were analyzed. However, there were strong positive correlations between the duplicate analyses for the samples. While these data are cross-sectional, the profile we reported is nevertheless highly consistent with that of international samples. Routine longitudinal monitoring, the analysis of larger samples of cannabis grown using known cultivation methods, and sampling from multiple parts of the plant would assist us in better understanding potency trends and the impacts of cultivation technique on cannabinoid profile.