This dual-focussed examination will critically compare and contrast the British Royal Army Medical Corps and the French Medical Service’s involvement in the Western Front chemical campaign between 1915 and 1918. Because the Anglophone historiography has tended to marginalise the French contribution to the allied chemical war, this article will attempt to re-balance the historical narrative by emphasising the collective nature and importance of this joint Franco-British enterprise. By interrogating a raft of under-utilised primary evidence in the French and British archives, this investigation will contribute to the ‘alliance literature’ by arguing that when it comes to aspects of the Franco-British chemical war such as the co-operation of the medical services, the appellation ‘together but alone’ does not fully hold. The article will explore avenues of the two national armies evolving process of mutual medical assistance, material exchange and scientific collaboration. The striking similarity of French and British gas casualty statistics is highlighted with reference to the overall congruence of their anti-gas strategies – notwithstanding the problematic nature of these statistics. In addition to enhancing soldiers’ resistance to the poison gas threat on the battlefield, the Medical Services were also responsible for the protection of local civilians in the war zones. With reference mainly to France, this investigation will discuss the significance and implications of the poison gas threat to the home front. Finally, the impact of chemical weapon production upon civilian war workers in France and Britain will be commented upon.