Evaluating the relevance of analyzing the aqueous phase and the suspended solids of influent wastewater (IWW) samples in the frame of monitoring cannabis consumption trends. Wastewater-based epidemiology (WBE) employs the analysis of human metabolic excretion products of xenobiotics in IWW with the aim of estimating their community use. Since cannabis is the most frequently used illicit drug, it is imperative to have accurate analytical methods available to measure cannabis biomarker Δ9-tetrahydrocannabinol (THC) and its major metabolites THC-OH and THC-COOH in IWW. Solid-phase extraction (SPE) is most commonly used as sample preparation method to isolate the biomarkers from the liquid phase of IWW. However, the analysis of the suspended solids and the application of the excretion rates in faeces are mostly ignored. To improve the estimation of cannabis consumption through WBE, both the aqueous phase and the suspended solids of IWW should be considered. In this study, liquid-liquid extraction (LLE) has been applied to obtain concentrations of the major biomarkers of cannabis (THC, THC-OH and THC-COOH) of the sorbed and dissolved phases of IWW. Using LLE, no filtration or centrifugation step is required prior to extraction in contrast with SPE and, therefore, the analytes present in suspended solids could be also simultaneously measured. LLE was applied to various IWW samples to extract both the full raw sample and the liquid phase obtained after centrifugation. In addition, extraction with a mixture of hexane-ethyl acetate was applied to the suspended solids separated from centrifugation of IWW. No significant differences were found when SPE or LLE were applied to the extraction of the liquid phase only, as both led to acceptable recoveries, 90% and 75% respectively. Therefore, both techniques could be used for the determination of cannabis biomarkers in the centrifuged liquid phase of IWW. After centrifugation of the samples, the separate analysis of the liquid phase and suspended solids in the IWW samples revealed that a significant fraction of the three biomarkers was present in the suspended solids (86% THC, 62% THC-OH, and 44% THC-COOH). In addition, the total amount of cannabis biomarkers observed by analyzing raw IWW on the one hand, and by separate analysis of the liquid phase and the suspended solids on the other hand, was in good agreement. Since SPE requires a centrifugation step, a significant amount of cannabis biomarkers present in the suspended solids is not measured. If SPE is used, a correction factor is needed to account for the significant sorption, but further research is required. Based on this new data; the added value, advantages and disadvantages of SPE in comparison with LLE should be reconsidered In WBE to estimate cannabis use, analysis of the suspended solids cannot be ignored. The sole analysis of the liquid phase by applying SPE would lead to a notable underestimation in cannabis biomarker concentration in raw IWW. Therefore, the analysis of the raw IWW by LLE offers more realistic information on the biomarker concentration in IWW leading to more reliable monitoring of cannabis consumption trends. Hence, the complexity of measuring cannabis biomarkers in IWW is better reflected. The preliminary results allowed to identify points to be addressed in future research to improve WBE back-calculations: i) sampling uncertainty related to solids, ii) partition of the cannabis biomarkers between liquid and solid phases, and iii) more accurate urinary and faecal excretion rates to use in consumption calculations.