The Hospital Authority Toxicology Reference Laboratory confirmed six cases of phenformin use, with or without complications, from July 2005 to November 2006. Two of the patients presented with potentially fatal phenformin-induced lactic acidosis. Phenformin was found (or suspected to be) adulterating Chinese proprietary medicine in five of the six cases. We report these six cases to highlight the underrecognised hazards posed by phenformin, a banned drug in Hong Kong.