Reduction in methamphetamine consumption trends from 2015 to 2018 detected by wastewater-based epidemiology in Dalian, China.

•Wastewater analysis was used to access temporal trends in methamphetamine use.•NH4-N as the biomarker was used to estimate the population size.•Obvious reduction of methamphetamine use was found in four-year monitoring.•The decreasing trends could be due to the strict crack-down on illicit drugs.