Pharmaceutical sludge is considered as a hazardous substance with high treatment and disposal fees. Anaerobic digestion could not only transform the hazardous substance into activated sludge, but also generate valuable biogas. This research had two objectives. First: studying the feasibility of anaerobic digestion and determining the biochemical methane potential (BMP) of pharmaceutical sludge under different Inoculum to substrate TS ratios (ISRs) of 0, 0.65, 2.58 and 10.32 in mesophilic condition of 37 +/- 1 degrees C. Secondly, investigating the removal efficiency of colistin sulphate during anaerobic digestion. The results showed that the use of anaerobic digestion to treat the pharmaceutical sludge is feasible and that it can completely eliminate the colistin sulphate. The highest biogas production from pharmaceutical sludge is 499.46 mL/g TS at an ISR of 10.32. (C) 2014 Elsevier Ltd. All rights reserved.