I find myself going through the poisons fast because I have them attached to my weapons and they have a high probability of infecting the enemy. If you do a lot of combat you will run through your inventory in a blink of an eye. This is a good reason to always make more than plenty of each kind you like to use. I tend to make 100+ for each weapon I use. You might be thrown off because you might not monitor it as you're trying to fight and when you peak back into your inventory you see it dwindle down fast.