Do poison effects stack?

If I use the same poison on a single target twice, do the effects stack?

For example I hit someone twice with a poison that does `1 damage per second for 10 seconds`, what are the results:

• `2 damage per second for 10 seconds`
• `1 damage per second for 20 seconds`
• `2 damage per second for 20 seconds`
• The timer resets back to 10 seconds on the last hit
• The second poison is ignored

This also leads to the question that if I use the `Concentrated Poison` perk in the `Alchemy` skill tree, are the effects the same as if I used two of the same poison in a row?

Lingering damage health does indeed stack. I tested this by giving multiple lingering poisons to an NPC via the poisoner perk. Her health went down noticeably faster when I gave her multiple doses. I do not know how the duration is affected as she died before the poison ran out. I will report back later after I have found a suitably healthy target.

After some testing, here is what I found out:

• All the potion effects are applied simulatenously. So if you have a poison that does 2 damage for 20 seconds and one that does 1 damage for 10 seconds, for the first 10 seconds, the target will take 3 damage per second, and for the next 10 seconds, he will take 2 damage per second. The end result will be 50 damage.
• The same poison will stack with each other, 2 poisons of 1 damage per second for 10 seconds will do 20 damage in total over 10 seconds.
• The reason why some people think they won’t stack is if you give an entire stack of poison to the target at one time (any amount >= 6), only the first poison in the stack will trigger. The rest will be given to the NPC and sit in their inventory as a potion that they now can use. If you want all the poison effects to trigger, you have to give it one at at time (and risk the 10% chance of detection at each attempt.)

This also leads to the question that if I use the Concentrated Poison perk in the Alchemy skill tree, are the effects the same as if I used two of the same poison in a row?

This is correct.