This is a pretty unclear portion of the rules. Potions which affect combat which can be played "at any time" are pretty self explanatory-- you have to be in an adjacent room or same to use it. What about curses and scrolls which affect combat? Some of them seemingly can impact combat without being near it.
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Thus far, the best use cases I've seen have been that scrolls and curses can be used at any time whatsoever even if you aren't in an adjacent room. You do have to read some of the cards carefully, as it may determine before or during a combat phase. Potions are still as they were, only adjacent or same room. With the portal expansion, potions can be used across portals.
There's an older discussion I found on Munchkin Quest forums that seems to back up this train of thought. Seems to also be backed by Steve Jackson.
I think you can use them at any time. Those are the house rules my friends and I play with.