In this study, effects of a chemical synthesis-based pharmaceutical wastewater on activity or acetoclastic methanogens within a lab-scale anaerobic completely stirred tank reactor (CSTR) were investigated. The CSTR was initially fed with glucose and loaded up to an organic loading rate of 6 kg COD m(-3) d(-1) at which 92% soluble COD removal efficiency was achieved. Acetoclastic methanogenic activity was found to be 336 mL CH4 gTVS(-1) d(-1). The CSTR was then, fed with several dilutions of preaerated wastewater with glucose at 10% wtv(-1), 30% wtv(-1), and 70% wtv(-1) ratios and followed by 100% wtv(-1) preaerated wastewater. In the latter, 71% soluble COD removal efficiency was obtained and a maximum acetoclastic methanogenic activity was 166 mL CH4 gTVS(-1)d(-1). Finally, raw wastewater diluted with preaerated wastewater was fed into the CSTR in increasing ratios of 10-60% wtv(-1). Total failure of anaerobic reactor was observed at 60% wtv(-1) raw wastewater fed. The study was, therefore, discontinued due to poor acetoclastic methanogenic activity of reactor sludge which was totally distorted.