Wastewater-based epidemiology (WBE) is an alternative approach for monitoring the prevalence of illicit drug abuse. The levels of methamphetamine and ketamine in the influent wastewater from eight wastewater treatment plants (WWTPs) during four months in 2019 were determined by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) after solid phase extraction. Mean population-weighed consumption of methamphetamine and ketamine was 188.6 and 9.1 mg 1000 inh-1 day-1 in City 1, respectively, while the corresponding consumption in City 2 was 57.0 and 7.6 mg 1000 inh-1 day-1. Spatial variations were observed between the two cities and among different WWTPs within a city. Evident decreasing trends of methamphetamine and ketamine consumption were observed from the first to the fourth sampling campaign, in which methamphetamine and ketamine consumption declined by 47% and 38% in City 1 and 44% and 83% in City 2, respectively. This study provides more comprehensive data on methamphetamine and ketamine consumption, which will help local anti-drug authorities identify hotspots of illicit drug abuse and take effective measures.