From Diablo Wiki
Poison damage in Diablo II is listed as damage over time (), and the actual damage done per second can be virtually impossible to calculate with the damage stacking from skills, weapons, charms, etc. Poison damage does not stack with multiple attacks, but resets each time to the most recent poison-based attack. Poison damage can kill, but it's not especially useful for that in large games, or past normal difficulty. It is very useful to keep monsters from regenerating hit points, since they will not heal while poisoned.