Current wastewater-based epidemiology (WBE) studies are predominantly focused on the analysis of urinary biomarkers present in the liquid phase of influent wastewater (IWW). This approach systematically underestimates less polar metabolites, such as cannabis biomarkers. These biomarkers can potentially sorb to and desorb from suspended particulate matter (SPM) present in IWW. This study investigates the bidirectional partitioning of THC[sbnd]COOH between the liquid phase and SPM of IWW by performing multiple sorption experiments using THC[sbnd]COOH-D₉ as a surrogate due to the unavailability of blank SPM and blank IWW. In addition, this study involves the analysis of IWW collected from eight wastewater treatment plants (WWTP) (n = 56) across four European countries, where raw IWW, the liquid phase and SPM were separately analysed to identify potential trends in the fraction of THC[sbnd]COOH in the SPM between and withing the examined locations. Based on the performed sorption experiments, bidirectional partitioning of THC[sbnd]COOH between the liquid phase and SPM was noted, showing partition between both phases when only one of the two phases was spiked. It was illustrated that the concentration of SPM had a notable influence on the THC[sbnd]COOH partitioning between both phases. In addition to the inter location variability (average THC[sbnd]COOH present in the SPM ranged 31–59 %), a substantial intra location variation was also observed, e.g., in one location ranged 17–58 %. While the determination of a correction factor for the amount of THC[sbnd]COOH present in SPM would be ideal, this is challenging, since the amount of SPM is not fixed in all IWW samples. Although SPM has influence on the THC[sbnd]COOH partition, no correlation (p value Spearman correlation = 0.3160) was observed between the SPM concentration and the fraction of THC[sbnd]COOH in the solid phase. Moreover, the collection of homogenized samples is difficult, and the time required to reach an equilibration in partitioning of THC[sbnd]COOH between both IWW phases remains unclear. Due to i) the large inter-and intra-location variation of THC[sbnd]COOH present in the SPM, ii) the variability in SPM concentration in IWW samples, and iii) the time required to reach a partitioning equilibration, an analytical procedure based on liquid-liquid extraction (LLE) that considers both the liquid phase and SPM of IWW is recommended to reduce the overall uncertainty for THC[sbnd]COOH measurement in IWW. It was illustrated that this extraction method is capable to recover the total concentration of THC[sbnd]COOH in both phases.