Related Rule
Practice Relating to Rule 72. Poison and Poisoned Weapons
In its written statement submitted to the ICJ in the Nuclear Weapons (WHO) case in 1995, Nauru stated:
Clearly it is a violation of customary international law to use poisons or other analogous substances. Thus even where a State is not a party to the Geneva Gas Protocol it is nonetheless bound under customary law to refrain from using poisonous weapons. 
Nauru, Written statement submitted to the ICJ, Nuclear Weapons (WHO) case, 15 June 1995, p. 11.