Recent cannabis debates have thrust the issue of cannabis potency back into the limelight. calls to put cannabis in Class A because of rising potency and health harms, to proposals to cap retail cannabis potency at 15% THC, the issue remains clouded with misunderstandings. Below we explore the potency issue; what does potency actually mean, how does it impact cannabis related risks, and how can it be sensibly regulated?

There is a certain amount of confusion around the concept of cannabis potency­ both ­what ­it ­means­ in ­technical ­terms,­ and ­what ­its ­implications­ are for the risks associated with the use of different cannabis products consumed in different ways. 

People are familiar with the concept of alcohol strength, expressed in percentage of alcohol content, and how this relates directly to the effects and risks of how much they consume. Standardised alcohol units are a good example. Key to their utility is that they can be readily translated between different alcohol products. However, the situation is less straightforward with cannabis, and cannot be directly compared for a number of reasons.

Cannabis potency is usually measured in terms of the percentage of its key psychoactive ingredient, \-tetrahydrocannabinol (THC or simply­ THC),­ but ­THC­ is ­only­ one ­of ­over­ 80 ­different ­cannabinoids­ found­ in ­the ­cannabis ­plant, ­key ­among ­these ­being ­cannabidiol ­(CBD).­ Because ­CBD­ interacts ­with ­and­ modifies ­the ­effects ­of ­THC,­ the ­ratio­ of the two is important not only as it shapes the nature of the subjective cannabis­ experience ­(CBD­ is­ thought­ to ­have ­a ­more­ sedative/calming ­effect), ­but­ also­ because­ CBD­ is­ thought­ to­ have ­anti-psychotic­ properties, ­potentially­ reducing the risks of psychotic episodes or psychotic illness related to cannabis use. The many other cannabinoids present in cannabis are less well understood but their relative proportions may also have subtle influences on the variable effects (and possibly risks) of different strains.

Lower-strength,­ outdoor-grown ­cannabis­ tends­ to­ be ­less­ than­ 10%­ THC,­ while (more commonly) indoor-grown, ‘premium’ cannabis varieties are predominantly in the­ 10-25% ­range.­ The­ potency ­of­ lower-quality ­and ­premium-grade­ resin has historically been roughly the same as this in European markets (but notably rising in recent years), although newer techniques such as solvent or carbon dioxide extractions have­ produced ­oils ­and­ other­ concentrates­, such ­as ­butane ­hash ­oil ­(BHO,­ the semi-solid forms sometimes known as ‘wax’ or ‘glass’)­, that­ have­ extremely ­high­ potencies,­ some­ reaching ­concentrations ­of­ over ­80%­ THC.

Another factor that complicates our understanding of cannabis potency is­ that­ the­ level­ of ­intoxication­ and­ the­ speed­ of­ onset ­of­ effects­, which­ will­ determine ­the ­subjective­ experience,­ depend ­in ­large­ part­ on­ the­ particular preparation, method of consumption, and using behaviours.

A ­given­ amount ­of­ cannabis ­can ­be­ smoked (or vaporised) ­in ­different­ ways­, in ­terms­ of how many inhalations the user takes, how deep the inhalations are and ­how ­long­ they ­are ­held ­in­ the ­lungs­, so ­the ­amount ­of ­active ­content­ that different individuals actually absorb can vary quite considerably. 

With smoked or vaporised cannabis, the onset of the effects is very rapid, meaning that individuals are able to dose control relatively easily. If they have not reached the desired effect, they will continue. If they have reached it, they can stop. On this basis, potency would seem to be less of a concern for­ inhaled ­cannabis ­use­, indeed ­higher-potency ­cannabis­ could­ mean­ fewer inhalations to achieve the same effect, thereby reducing respiratory risks. However, while such ‘auto-titration’ dose control behaviour is the norm, higher-potency cannabis can still potentially lead to higher total consumption and correspondingly pose greater risks. 

With more potent varieties a large dose of active content can be received in a single inhalation, and the larger such individual doses are, the harder it becomes to fine tune dosage control, meaning the potential to consume more than planned or desired is increased. This is particularly the case for novice users.

This risk of consuming more than planned (with potentially negative or undesirable effects) will be amplified when the potency of the cannabis being consumed is unknown. However, this problem can be reduced or effectively eliminated in a properly regulated system in which:

• Buyers ­are­ able­ to­ choose ­from ­a ­range­ of­ clearly ­labelled ­products­ of­ different potencies.
• Buyers­ are­ able­ to ­take­ guidance­ from ­licensed, ­trained ­vendors.
• There ­is ­relevant ­information ­on ­dosage,­ effects ­and ­safer ­use­ at­ point­ of sale and on all packaging.

The increasing average potency of illegal cannabis is a genuine observed phenomenon in the US, and to a lesser extent in Europe, although what appears to be a long-term incremental rise in average potency has provoked many exaggerated ‘reefer madness’-style claims that have little basis in reality. In Western markets at least, the increasing market dominance of indoor-grown ‘premium’ cannabis, combined with likely actual increases in its potency (through selective breeding and developments in intensive growing technologies), has probably pushed average potency up to between two to three times what it was in­ the­ 60s­ and­ 70s.­ Such­ averages­ do,­ however, disguise a great deal of variety within markets and between different localities. 

There was of course very potent cannabis (particularly in resin form)­ available­ in ­the ­60s­ and­ 70s, ­so ­the­ suggestion that what is being consumed today is a ‘completely different drug’ is misleading: the observed trend is primarily due to there being a greater proportion of more potent varieties on the market.

To moderate use of cannabis and prevent adverse health effects associated with high-potency products, policymakers may consider capping potency for all products, including edibles, concentrates, extracts and flower, by setting a limit on the percentage or total amount of THC in a given product sold in the U.S. States would be allowed to set their own stricter limits. Potency caps are generally only considered for edibles in the U.S., although a few states also cap some concentrates. Of the 19 states and D.C. that have legalized nonmedical cannabis use by adults, Vermont and Connecticut are the only states with a potency cap on both flower and concentrates. Both states limit the percentage of THC to 30% in flowers and 60% in solid concentrates sold. Both states, however, exclude prepackaged vape pens or cartridges from these potency limits. 

Throughout 2021, policymakers in other states, including Colorado, Washington, Montana, Massachusetts and Florida, introduced or drafted bills to similarly limit THC levels. These bills have either failed or been shelved until the following legislative session, primarily due to resistance from cannabis businesses and industry proponent groups, arguing that potency caps on the legal market will impact a large proportion of products and lead to diversion to the illegal, unregulated market. The inability of state policymakers to institute potency caps in markets that are already operating highlights the importance for federal proposals to include this policy at the onset of potential legalization.

THC potency caps on flowers and other products are far from novel in other jurisdictions that have legalized cannabis for recreational use. Both Uruguay and Canada have adopted caps on potency for some products. In Uruguay, the maximum THC content across all products sold in pharmacies was initially set at 2% in 2013 and later increased to 9% in 2017. Extracts and edibles are prohibited, and only five companies are authorized to produce and distribute nonmedical cannabis. 

In comparison, the average potency is higher within Uruguay’s cannabis social clubs and home growers, where no potency limit exists, suggesting the potential effectiveness of this policy tool. Further, Uruguay’s cannabis regulatory agency (Instituto de Regulación y Control del Cannabis) estimates that the legal market accounts for over 50% of the country’s total cannabis users, indicating that the legal market has displaced a sizable portion of the illegal market even with strict federal regulations.

In Canada, the maximum THC content is 10 mg/package for edibles, 10 mg/unit or dispensed dose for ingested extracts, with a maximum number of 1,000 mg per package of either an ingested or inhaled extract, and 1,000 mg/package for topicals.

The potency of flower material is not officially capped. The most recent data show the average THC potency for flower in British Columbia’s legal market is 19%, comparable to that sold in the U.S., but higher than that reported for Canada overall. This suggests that the average potency of cannabis flower may be on the rise in Canada.