Sustainable operation is an essential challenge in many municipal wastewater treatment plants. Among many types of wastewater mixed in a sewer, healthcare wastewaters need special attention due to their hazardous substance content, which can be toxic to activated sludge. This study compared the acute inhibitory effects of healthcare wastewaters (HW) and medical laboratory wastewater (MLW) on conventional activated sludge (CAS) and membrane bioreactor (MBR). The sensitivity test showed that nitrifying bacteria (NBs) in MBR sludge have higher resistance to acute toxicity than the CAS. Compared with HW, MLW caused much higher inhibition on both sludges. When the ratio of HW in the tested domestic wastewater was 10%, inhibition of NBs was 39% in AS, while it was 31% in MBR. When the ratio of MLW in the tested domestic wastewater was only 10%, 72% of NBs in AS and 57% of NBs in MBR were already inhibited. The higher resistance of NB in MBR may be explained by the diversity of microorganisms in the MBR operated at high sludge ages. The findings of this study may be used to estimate the acute inhibition effect of HW and MLW discharged directly to the sewage at higher loads.