According to a Liberty Tree article, SPANK is a white powder that is normally snorted and is extremely expensive, very similar to cocaine which is probably used as an ingredient. It can also be placed up the user's rear and, among more serious users, taken as a suppository (which might explain the name of the drug). It takes a serious toll on the user's sanity, similar to meth. One of the first people to try it was notable house and dance music DJ Boy Sanchez in 1998.
Mission Appearances in GTA III
Closeup of a SPANK bomber in "Kingdom Come".
A coffee stall in the mission "Espresso-2-Go!".