Wastewater study sheds light on tracking cannabis use challenges

By Tarun Sai LomteReviewed by Susha Cheriyedath, M.Sc.Apr 3 2024

In a recent study published in the journal Water Research, researchers discussed the progress in wastewater-based surveillance (WBS) of cannabis use.

Cannabis is the most used drug globally, and its frequent use is associated with anxiety, depressive disorders, brain development interference, and social/legal consequences. Moreover, cannabis consumption can strain healthcare systems and legal frameworks. Notwithstanding these concerns, cannabis is being increasingly legalized for recreational and medicinal purposes.

Making waves: Wastewater-based surveillance of cannabis use. Image Credit: SpiritArt / Shutterstock

Preliminary signs/results of this legalization indicate increased daily intake and tax revenues and decreased arrest rates. This has also enabled the implementation of harm-reduction strategies. Thus, continuous monitoring of cannabis use is crucial to gain insights into spatiotemporal trends and assess the impact of new enforcement measures, legal status, and harm reduction initiatives. WBS and wastewater-based epidemiology (WBE) have become popular for public health monitoring.

WBE monitors changes and spatial differences in drug use, complementing established indicators. Thus, it is a vital source of information on community-level drug consumption. Nevertheless, there have been uncertainties with WBE-derived estimates of cannabis use than other drugs. The present study discussed the progress in WBS of cannabis use and existing challenges.

Analytical barriers

The analytical techniques in WBS involve determining diverse biomarkers, where 11-nor-9-carboxy-Δ⁹-tetrahydrocannabinol (THC-COOH), the primary urinary metabolite of the principal psychoactive compound in cannabis, tetrahydrocannabinol (THC), is just one of the analytes to be measured. A single analysis of multiple analytes is time-saving, economical, and more efficient, albeit a compromise of experimental conditions is required. Furthermore, analyzing THC-COOH requires pH adjustment.

At acidic pH, the hydrophobic molecular form of THC-COOH favors adsorption to sampling/processing material surfaces or particulate matter. Thus, a best-practice analytical protocol was proposed to avoid acidification. Nevertheless, the lipophilic features of cannabinoids impede analytical determination. As such, more specific techniques have been introduced to detect THC and its metabolites in influent wastewater solids.

Challenges due to in-sewer and sample dynamics

Quantification of THC and metabolites in suspended solids and aqueous phase of influent wastewater showed their partitioning in the two phases of the influent. Further, it is speculated that fecal matter from non-users and users may impact the total load in the aqueous phase. These insights have opened additional inquiries, unveiling knowledge gaps warranting further analyses on the impact of influent composition, sampling uncertainty related to the collection of solids, and partitioning dynamics during in-sewer transport.

Pharmacokinetic barriers

Interestingly, WBS can estimate drug consumption via a back-calculation using influent mass loads, pharmacokinetic excretion factors, and molar mass ratio of parent/metabolite. Distinct excretion correction factors (36.4 and 182) derived from urinary levels of THC-COOH have been proposed for cannabis. While these correction factors accounted for uncertainty, they do not consider (other) metabolites excreted in feces. As such, more investigations are required to corroborate and increase knowledge of urinary and fecal excretion (of THC-COOH).

Administration routes

Different administration routes of cannabis yield distinct excretion rates, affecting back-calculations. Although smoked cannabis is the most common route, the use of vaping and edible cannabis has been increasing. In 2020, more than one-fifth of 12^th-grade students tried vaping cannabis, which is twice the estimate from five years ago.

Such changes can vary by country and time and could be related to cannabis legalization. This can also lead to changes in the type of cannabis used; for instance, cannabidiol has increasingly become popular. Therefore, there is a need for techniques that can distinguish between analytes when other cannabis types become prevalent.

Current state of WBS of cannabis

WBS is an instrumental tool that can report high-resolution trends and changes. However, currently, careful interpretation of WBS data is necessary to evaluate cannabis consumption. Some researchers have described how best to utilize these data. Caution should be exercised for spatial comparisons of excreted quantities of THC and metabolites in the influent.

Influent wastewater and sewer catchments exhibit variations across locations; consequently, spatial trends may be susceptible to greater uncertainty. Further, cannabis use trends derived from WBS have an inherent temporal value when catchment demographics remain unchanged. The relative scale of drug use is often essential to identify problem areas and make policy decisions.

Concluding remarks

Taken together, the study highlighted the current barriers to accurate cannabis consumption estimates. The authors propose that future research should better understand administration and clearance routes, integrate fecal data into correction factors, and develop a consensus for a common approach to report estimates from WBS data. They also suggest caution while drawing conclusions from WBS data until these challenges are addressed.