Inhaling vapors from a potion of gain level produces a message like “You smell hard work” or “You smell elbow grease” (perhaps if hallucinating), granting you 10% of the experience points needed to advance to the next level. Monsters may be similarly affected.

Note that the 10% is calculated just based on the level thresholds and your own experience points don’t matter (only your current level). If you are level 5, with level 5 reached at 120 points and level 6 reached at 200 points, you will always get 10% of (200-120) = 8 points.