The ratio between CLI and its metabolite dmCLI was higher than the other AAs/metabolites observed in this study at 7.10 ± 2.51 and 7.20 ± 3.67 for Bath and Keynsham, respectively. A previous study has demonstrated a lack of stability for these compounds, with degradation of over 80% reported for both CLI and dmCLI in wastewater after 24 hours . As degradation was similar for both CLI and dmCLI, it might be theorised that the ratios should have been lower. Stability could be an important factor here, as the standard deviations are large. However, as with MTZ, CLI creams can be applied topically, as a first-line acne treatment and for the treatment of bacterial vaginosis. Whilst MTZ was shown to be predominantly prescribed as oral treatment, compared to topical; this was not the case for CLI, which observed higher amounts of topical prescriptions (figure ). Bath reported an average of 0.11 kg/month of oral prescriptions versus 0.052 kg/month of topical prescriptions. Keynsham reported 0.096 kg/month of oral versus 0.044 kg/month of topical prescriptions. Whilst, in general, the monthly oral prescriptions outweighed topical, November and December 2019 in both catchment sites observed similar prescribing for both (~0.059 kg/month for both Bath and Keynsham). This could therefore also cause higher levels of parent compound in comparison to the metabolite, hence could also push this ratio up. Whilst contribution from topical AAs could be low, understanding the breakdown of prescription data (topical versus oral) is still important when back calculating consumption at the community level. For other AA/metabolites that have previously demonstrated good stability in wastewater, e.g. CLR and dmCLR, concordance in the ratios between the two catchment sites is observed. The importance of biomarker stability is therefore highlighted again, especially for comparing between different communities. Different catchment areas will likely have unique conditions in the sewage pipes which could be facilitating faster degradation or transformation of either parent or metabolite. Again, more studies on fate of AAs and metabolites from source to WWTP could provide invaluable insight to these potential processes. However, by comparing the ratios of stable biomarkers between different catchment areas, WBE has promising potential to provide highly accurate data and comparisons. Statistically significant values are in bold.