Related Rule
Practice Relating to Rule 72. Poison and Poisoned Weapons
In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, Sweden stated: “As far back as the 17th century, Hugo Grotius stressed that poisoning was not allowed under international law. In certain respects, the principle of the prohibition of toxic weapons has also been codified (chiefly as a result of the 1925 Geneva [Gas] Convention).” 
Sweden, Written statement submitted to the ICJ, Nuclear Weapons case, 20 June 1995, p. 5.