When I go to Meditation to drink a potion, if I have more than 1 bottle of the same potion then they are grouped together and I cannot help but drink all of them at once! So when I click on a potion - it puts all 5 of them in the triangle thingy. But I only need to use 1 at a time - how do I separate them by single portions so that I have 4 bottles remaining instead of being forced to drink them all at once?

You can't drink 2 of the same potions, as far as I know.

Visually, it will appear as if you have placed them all in the drink slot. But, it will only use one potion, so don't worry.

They just disappear from the list because you are already going to drink it.

But I can and I do! I had 5 Rook potions and after drinking the bottle there were no more left in the list and the potion time was not that of 1 bottle but that of 5 bottles!

You mean you have 50 minutes of potion effect??

Since I"m always overlapping Cats, I know for a fact that potions simply over-right the time, not add to it.

I also know that potions get dropped as a stack into the drink part of the UI, but only one is used up.

So, if by some strange reason, your game is a unique snowflake and does things differently, I think a few screenshots illustrating this problem would help.

You were correct and I am an idiot - after clicking on the Rook stack (which I have 5 of) they, all the remaining 4 of them, disappeared from the list of the potions, and I drank it and got 10 minutes of effect (1 bottle worth). But when I went back to the Meditation menu the 4 remaining Rooks re-appeared as if by magic.
So I'd say this is a poor thought out design moment because it's confusing the player. When we are in that menu with the list of potions, and when we select one from the stack - the rest should remain there!

So thanks everyone for replies, I guess that answers it - the potions from the stack remain and are visible again only after you return to the Meditation.

It may be done on purpose, because you're not supposed to be able to drink more than one of a kind, each time.

I'll try some testing....

[edit] Yup. The Inventory empties of whatever potion you use, so you can't take more than one of a type at a time. If you exit, then re-enter meditation right away, you can drink the same potion again, but it simply "overwrites" the first one. You can't get double-effect or double-time from the same potion.