A poison potion is simply a highly toxic liquid in a potion flask. 

Typically, poison potions are odorless and of any color. 

Ingestion, introduction of the poison through a break in the skin, or possibly just skin contact, will cause death. 

Poison can be weak (+4 to +1 on saving throw), average, or deadly (-1 to -4 or more on the saving throw). 

Some poison can be such that a neutralize poison spell will simply lower the toxicity level by 40% - say from a -4 to a +4 on saving throw potion. 

You might wish to allow characters to hurl poison flasks.