Two comments:
1) The word "essence" suggests something highly concentrated, rather than something more diluted.
2) How about delaying the effect of the potion. For example, have it take effect a minute after drinking, or even an hour after drinking, for some potions. Or you could make it somewhat unpredictable. The character may not be able to predict exactly when it will take effect. (You could also use this for normal potions that the players might buy or find--slightly defective potions!)
I like the comment made by someone else that you could have it take up to a minute to drink. Another variation on this would be to make it like a tea that the player would have to allow to brew before drinking. It comes in the form of a tea bag. If a player brews it up ahead of time and doesnt drink it immediately it would begin to lose its potency.
Thanks for the creative ideas! I think I'll use something like this in my campaign.