So, I find this to be very convenient. This macro will put the poison on your main-hand weapon if you left-click it, or your off-hand weapon if you right-click it.
But the good part is that if you already have poisons on, and you are switching poisons, or just reapplying them because they are about to fall off, it skips the "Do you want to replace X poison with Y poison?" pop up.
#showtooltip
/use Deadly Poison IX
/use [button:1] 16; [button:2] 17
/click StaticPopup1Button1
Of course you just replace the poison's name with whatever poison you want.
I have a macro for each of my poisons.
Enjoy. :-)