Geralt is given this potion by the Visionary during the quest "The Path to Vision". He must go to "Sabrina's pupils", two lakes at the end of a ravine north west of the Kaedweni camp where there is a shrine to Sabrina and drink the potion, and then return and describe the vision he has to the Visionary.
Geralt dreams of a giant chicken appears to be surrounded by an every growing number of wooden dildos (or mushrooms, for the faint of heart). He mutters something to himself about no one ever needing to know about that vision after it ends.
When he returns to the visionary, he can be honest about the vision, or lie.