In January, 2018, California became the eighth state in the USA to introduce the sale of recreational cannabis. Despite this milestone, the absence of a standardised cannabis unit continues to impede research investigating the harms of smoking cannabis.

1 Englund A

Freeman TP

Murray RM

McGuire P Can we make cannabis safer?. In the absence of a standardised cannabis unit, researchers treat all cannabis cigarettes (joints) equally, or ignore quantity and focus instead on frequency of use. Joints vary in size and in potency. Thus, two people who smoke one joint per day are not likely to consume the same dose of the psychoactive ingredient tetrahydrocannabinol, and their consumption will differ from someone who smokes ten joints per day. Failure to acknowledge these differences prevents the establishment of clear definitions for problematic cannabis use and hinders the medical community's capability to make public health recommendations on using cannabis safely.

2 Hindocha C

Freeman TP

Curran HV Anatomy of a joint: comparing self-reported and actual dose of cannabis and tobacco in a joint, and how these are influenced by controlled acute administration. The standard drink unit paved the way for guidelines on binge drinking and low-risk alcohol use. In standard drink definitions, grams of ethanol are used as a direct measure of potency. However, grams are difficult for individuals to estimate. Different volumes, dependent on the type of alcohol, are defined within the guidelines to meet a threshold for one standard drink. Thus, individuals only report the number of shots of straight spirit, cans of beer, or glasses of wine they have consumed. A similar strategy for quantifying cannabis would be beneficial because people who use cannabis often have difficulty estimating the amount of cannabis they roll in joints.

3 Mariani JJ

Brooks D

Haney M

Levin FR Quantification and comparison of marijuana smoking practices: blunts, joints, and pipes. 4 Norberg MM

Mackenzie J

Copeland J Quantifying cannabis use with the timeline followback approach: a psychometric evaluation. 5 Tomko RL

Baker NL

McClure EA

et al. Incremental validity of estimated cannabis grams as a predictor of problems and cannabinoid biomarkers: Evidence from a clinical trial. The use of a substitute will improve quantification accuracy, particularly as research continues towards the development of a standardised cannabis unit. Mariani and colleaguesweighed how much oregano (as a cannabis substitute) participants placed on rolling papers and cigar wrappers. Subsequently, Norberg and colleaguesshowed that the substitute method is reliable and valid, and Tomko and colleaguesreported that it is a better predictor of cannabinoid biomarkers than frequency alone and more simple measures of quantity (ie, number of joints per day).

However, implementing this strategy has limitations because substitution only quantifies cannabis derived from dried plant material and the cannabis weight is not equivalent to the dose of tetrahydrocannabinol or cannabidiol consumed.

In moving toward the development of a standardised cannabis unit, we recommend assessing cannabis potency in terms of three ratios of tetra-hydrocannabinol and cannabidiol: varieties with high concentrations of tetrahydrocannabinol and low concentrations of cannabidiol, varieties with concentrations of tetra-hydrocannabinol and cannabidiol that are roughly equal, and varieties containing low concentrations of tetrahydrocannabinol and high concentrations of cannabidiol. This method might help to determine a standardised cannabis unit in grams for each ratio—eg, 1 standard cannabis unit equals 0·25 g of a variety with high tetrahydrocannabinol and low cannabidiol concentrations, 0·5 g of a variety with equal tetra-hydrocannabinol and cannabidiol concentrations, and 0·75 g of a variety with low tetrahydrocannabinol and high cannabidiol concentrations.

Accounting for this variation in dose of tetrahydrocannabinol and cannabidiol consumed might help differentiate users at high versus low risk for problematic use.

RLT reports a grant from the National Institute of Drug Abuse (NIDA R01DA042114). CH and MMN declare no competing interests.

Article Info Publication History Identification DOI: https://doi.org/10.1016/S2215-0366(18)30088-9 Copyright © 2018 Elsevier Ltd. All rights reserved. ScienceDirect Access this article on ScienceDirect