Cytostatic drugs, mainly used as chemotherapy compounds, can pose serious threats to aqueous ecosystem and human health once released into the natural environment. We investigated the use of an anaerobic osmotic membrane bioreactor (AnOMBR) for removing cytostatic drugs from wastewater. The AnOMBR utilizes a dense forward osmosis (FO) membrane in an anaerobic digester with prolonged sludge retention time (60 days). The high rejection of the FO membrane combined with the extended organic retention time in the reactor ensured high removal rates (more than 95.6%) for all the eight cytostatic drugs investigated. With regard to their removal routes in the AnOMBR, the eight cytostatic drugs can be divided into several groups. Doxorubicin, Epirubicin and Tamoxifen were nearly completely removed through the adsorption of anaerobic sludge, while Methotrexate and Cyclophosphamide were mainly removed by biodegradation and FO rejection, respectively. In addition, Mitotane, Azathioprine and Flutamide were removed by both biodegradation and adsorption. This work provides critical insights into the removal mechanisms of high-retention AnOMBRs.