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Zimbabwe
Practice Relating to Rule 72. Poison and Poisoned Weapons
In its oral pleadings before the ICJ in the Nuclear Weapons case in 1995, Zimbabwe fully shared the analysis by other States that “the threat or use of nuclear weapons violates the principles of humanitarian law prohibiting the use of weapons or methods of warfare that … utilize poisonous or analogous substances”. 
Zimbabwe, Oral pleadings before the ICJ, Nuclear Weapons case, 15 November 1995, Verbatim Record CR 95/35, p. 27.
The Report on the Practice of Zimbabwe states that the prohibition of the use of poison is part of customary international law. 
Report on the Practice of Zimbabwe, 1998, Chapter 3.2.