As cannabis becomes more available and socially accepted in the U.S., it is increasingly important to facilitate research on this drug’s effects. A major hindrance, however, has been the lack of a standard unit by which to measure cannabis intake and compare its effects across studies. Existing experimental data are often hard to interpret due to the wide variability in potency of cannabis plant material and extracts, the lack of standard measures of use, and the wide variety of ways people consume cannabis. To help rectify this, NIDA, along with the National Cancer Institute; the National Heart, Lung, and Blood Institute; and the National Institute of Mental Health, have published a notice in the NIH Guide directing researchers funded by these institutes to measure and report their findings from clinical research on cannabis using a standard unit of delta-9-tetrahydrocannabinol (THC) of 5 milligrams.
A standard unit is not a limit, nor any kind of recommendation for consumption that would apply to consumers or to dispensaries; it is simply a unit of measure to help facilitate cannabis research. Similar standard measures have also been applied for other substances. Researchers use morphine milligram equivalents to compare effects of opioids having widely varying potencies. And research on alcohol and tobacco has been facilitated by defining a standard drink (.6 fl oz or 14 grams of pure alcohol) and a cigarette, respectively.
Cannabis is a complex plant with many constituents that might influence its effects; however, research has established that THC is the main chemical responsible for the high that users seek as well as for some of the medicinal effects that have been demonstrated in clinical trials. Like other drugs, THC’s effects vary based on the route of administration and the tolerance of the user, among other factors. Having a standard unit of measurement will make it easier to compare the influence of these factors on how individuals respond to the drug.
A standard unit does not place a limit on how much THC researchers can use in experiments—they can use multiples (or fractions) of the unit. But adoption of a standard unit for measuring and reporting purposes will facilitate data interpretation and will make it possible to design experiments on drug effects that have real-world relevance, as well as make it easier to translate that research into policy and clinical practice.
Utilizing a THC standard unit in cannabis research will help us gain a better understanding of the effects of cumulative THC exposure, such as the effects of prenatal and/or adolescent exposure on brain development, cognition, and educational attainment. It will also facilitate understanding THC’s adverse medical effects seen in frequent users such as hyperemesis or cardiovascular toxicity, as well as effects like psychosis seen in individuals with certain underlying vulnerabilities. In research on treatment of cannabis use disorder, a standard unit will enable researchers to more accurately capture reduction in use as an outcome measure, the same way researchers now capture clinically meaningful reductions in alcohol consumption via heavy drinking days, defined as four or more drinks per day for women, and five or more for men. In the cancer setting, the use of a THC standard unit will aid research investigating molecular mechanisms of cannabis effects on tumor growth, invasion, metastasis, as well as clinical research to determine benefits and risks of cannabis use for a variety of cancer treatments side-effects such as nausea and vomiting, pain management, neuropathy, anxiety, insomnia, and loss of appetite.
A few years of information-gathering and deliberation have gone into NIDA’s decision to establish a 5-milligram standard unit of THC. The Cannabis Policy Research Workgroup established in 2017 by the National Advisory Council on Drug Abuse issued a report in 2018 recommending that NIDA explore establishing a standardized THC unit to help researchers analyze cannabis use and to help those who use cannabis understand their consumption of this drug. In October 2019, researchers Tom P. Freeman (University of Bath, UK) and Valentina Lorenzetti (Australian Catholic University) proposed adopting 5 milligrams as the THC standard unit in an article in the journal Addiction. In March 2020, NIDA issued a request for information (RFI) from the research community, interested stakeholders, and the general public asking for input on the proposed establishment of a standard unit for cannabis research.
The responses reflected a diversity of opinions, but overall, there was support for the idea. Some suggested that a larger amount of 10 milligrams would be more relevant for people who use cannabis frequently or who use today’s high-potency products. But extensive discussion with experts in the field showed wider support for Freeman and Lorenzetti’s original recommendation of 5 milligrams. This dose may produce a high in both experienced and occasional users, but in most studies, has not produced adverse effects; and in some states, 5 milligrams is already the standard serving size in edible products that contain THC (others use 10 mg).
The 5-milligram standard unit will only apply to THC, not to other psychoactive constituents of the cannabis plant such as cannabidiol (CBD). Our hope is that adopting this 5-milligram standard will enable a clearer understanding of the effects of THC by researchers as well as the wider public.