two ideas for poisons that would be interesting to see used in combat.
Victim gets +2 on ALL rolls for 30 seconds (however many rounds that would be in your particular edition), after which must make a save. On a success, the victim loses half his HP and Strength points. On a failure, his heart explodes and he dies.
Lasts 1d6 hours. During that time, the victim takes damage from horrendous pain equal to half the "natural" result of any dice rolled. A successful initial save will halve all damage taken.