Related Rule
South Africa
Practice Relating to Rule 72. Poison and Poisoned Weapons
South Africa’s LOAC Manual (1996) expressly prohibits the use of poison. It lists poison among “certain weapons … expressly prohibited by international agreement, treaty or custom”. 
South Africa, Presentation on the South African Approach to International Humanitarian Law, Appendix A, Chapter 4: International Humanitarian Law (The Law of Armed Conflict), National Defence Force, 1996, § 34(f)(iii).
The manual further provides that “making use of poisoned … arms or ammunition”, as well as the “poisoning of wells or streams”, are grave breaches of the law of war and war crimes. 
South Africa, Presentation on the South African Approach to International Humanitarian Law, Appendix A, Chapter 4: International Humanitarian Law (The Law of Armed Conflict), National Defence Force, 1996, §§ 39(a) and (g) and 41.
South Africa’s Revised Civic Education Manual (2004) states:
i. Prohibited Weapons. The following weapons have been prohibited:
(4) Poison or Poisonous Weapons. 
South Africa, Revised Civic Education Manual, South African National Defence Force, 2004, Chapter 4, § 56(f)(i).
The manual further provides that “making use of poisoned … arms or ammunition” and the “[p]oisoning of wells or streams” are grave breaches of the law of armed conflict and war crimes. 
South Africa, Revised Civic Education Manual, South African National Defence Force, 2004, Chapter 4, §§ 61(a), 61(g) and 57.
South Africa’s ICC Act (2002) reproduces the war crimes listed in the 1998 ICC Statute, including “employing poison or poisoned weapons” in international armed conflicts. 
South Africa, ICC Act, 2002, Schedule 1, Part 3, § (b)(xvii).
In its judgment in the Basson II case in 2005, the Constitutional Court of South Africa stated:
[179] … There can be no doubt that the use of instruments of state to murder captives long after resistance had ceased would in the 1980s, as before and after, have grossly transgressed even the most minimal standards of international humanitarian law.
[180] The same has to be said of the use of poison to bring about the death of opponents … Such means of warfare are abhorrent to humanity and forbidden by international law. The use of poison to eliminate opponents in armed conflict has long been prohibited. 
South Africa, Constitutional Court, Basson II case¸ Judgment, 9 September 2005, §§ 179–180.