As said above, it's all about choosing your targets wisely. I find knocking down the strength on creatures to be most useful as it slows them down, allowing me to get more shots off without getting into a direct fight, which is good for killing creatures that you wouldn't normally be able to handle.
Having said that, when you are with powerful groups, or are fighting a group of creatures, the effects become less obvious, usually because either side is dead before you get much of a chance to observe the poison in action
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