I know this is almost a year old, but for anyone that stumbles across this post might find this information helpful if they're also stuck.
There are two ways to loop in UOSteam, you can use the "loop" checkmark in the script tab (not recommended), or you can do for or while loops (recommended). In this case, you want to perform your poisoning until it is the case that you're either dead (can't gain or perform actions while you're dead), or you have gained 100 poisoning (No need to waste any materials after that). There are of course other cases where we'd want to quit our loop, like if we don't have anymore poison potions, but we can use the "break" statement for that.
Another point is that, instead of hardcoding serial numbers, it's more helpful to use promptalias to have the player target what they want to use. This allows the script to be used universally between characters without being changed.
The version you presented is also only able to deal with that specific poisoning potion. You may want to include the different poisoning potions by searching for specific ones using the findtype nested within an if statement with all of the different poisoning levels. You'd want to only include the poison potion selection part inside the if statement as everything else remains the same.
Our code then would look something like this:
//Example Poisoning Trainer by Vlek, Leader of the Bloodskulls
promptalias 'objtopoison'
if not @findalias 'objtopoison'
sysmsg 'Please restart the script and select an item to poison'
stop
endif
while not dead and skill 'poisoning' < 100
useskill 'poisoning'
waitfortarget 15000
//This is only good for THIS specific poison level.
//You either have to change it manually, or make your
//script smarter to determine which to use based on your
//poisoning skill level. (if skill 'poisoning' < 50), etc.
targettype! '0xf0a' 'any' 'backpack'
waitfortarget 15000
target! 'objtopoison'
pause 750
endwhile
