Something similar happened to me and if you have made at least one mongoose with the ostmurk as an ingredient (that is it appeared in the formula ingredients tree) then it does not matter (you should see in your inventory that you don't have the ostmurk ingredient anymore) My recommendation is to go through with fighting the Kayran this is, go talk with the sorceress and tell her you are ready, save before doing that and save again just before fighting the kayran (once you are a few steps under the broken bridge, then go to meditation there and drink the mongoose (and any other potion you think might give you an edge), which only protects you against poisoning, you still have to dodge it's tentacles.
Hope this helps.
So I've been playing Witcher 2 since release day and very happy with it so far. However I got up to a point in the main quest where I can't progress any further. I'm doing the Kayran quest and I have to make a Mongoose potion using Ostmurk. I took the Ostmurk before the quest started and I managed to make one after Triss gave me the Formula using ingredient bought from the witch to fill in the blanks. But even though I havenot one but two Mongoose potions, the game doesn't acknowledge it and still says I need to make one(ie 0/1 Mongoose Potion in the quest objective) There's probably some trick I have to do. Can anyone tell me what to do to get past this?