Illicit drugs and drugs of abuse are emerging contaminants in the aquatic environment.Its presence in the ecosystem is an important concern and therefore it is necessary toinvestigate why these pollutants reach the aquatic environments after its treatment inwastewater treatment plants. There is a lack of information about the occurrence ofthese Illicit Drug in natural waters that is why is interesting to evaluate the factors thatcan influence the removal of Illicit drug in WWTP in the county of Halland, Sweden.The drugs that have been assessed in this paper are cocaine (COC), its metaboliteBenzoylecgonine (BE), MDMA, Amphetamine (AMPH), Methamphetamine (METH),and Cannabis (THC/THC-COOH)). The results show that AMPH and METH are theones who have higher concentrations in the effluents because just the eliminate 11,78%is removed from the water after treatment. Activated sludge methods are the besttreatment but factors such as residence time and sludge time influence in its efficiency.