This paper examines the historical construction of the boundaries between definitions of what counts as a ‘ drug’ and what counts as a ‘medicine’ by reference to two sets of substances, those related to opium and those related to tobacco and its active principle nicotine. It seeks to distance itself from ‘Whig’ understandings which see a substance focussed ‘ progress of scientific understanding’ as an explanatory framework of changes in concepts over time. Through a comparison of the different historical trajectory of definitions for the opiates and for tobacco, in particular in relation to the concept of addiction, it underlines the complex interaction between scientific constructs, policy alliances , consumption and culture and the relationship with technology. These have helped determine changing places.