The Unit of Cannabis: from a systematic review to a NIDA recommendation

In contrast to other, in particular, legal substances, cannabis use is associated with manifold peculiarities (e.g., various modes of administration, more than one psychoactive constituent, large variation of potency) which make the measuring of quantities of cannabis for personal use a real challenge. Also, the lack of a standardized unit makes clinical and epidemiological studies relating the use of cannabis with its consequences difficult to interpret. To try and solve that, the US based National Institute of Drug Abuse (NIDA) endorsed to use a standard unit of 5 mg of delta-9-tetrahydrocannabinol (THC) for research purposes. However, it remains unclear how researchers have previously unified cannabis use quantities.

We systematically reviewed Pubmed and Web of Science databases up to June 2021 (registered on PROSPERO, CRD42021261679). The keywords include cannabis, marijuana, hashish, standardization, quantity, grams, standard unit, and joint unit. We identified 31 suitable references (from 2006 to 2021), which proposed 21 different consumption units. In 16 studies, units were defined in grams of cannabis (from 0.14 g to 1 g in a joint). 9 of the units include equivalences of quantities across different consumption modes, and one includes information about potency (% of THC). Three studies expressed cannabis use quantities in the amount of THC (from 5 to 10 mg of THC). In another study, a single unit was defined by both quantity of cannabis (0.25 g) and THC (7 mg). In the studies identified, cannabis quantities were mostly defined using grams of cannabis. Units based on doses of its main psychoactive constituent, THC, dominate the more recent literature (in line with the NIDA’s publication). Further studies must confirm its utility in advancing on the knowledge about cannabis use consequences.