Potion of gain level
|Base price||300 zm|
|Monster use||May be used by monsters as misc items.|
Quaffing a non-cursed potion of gain level will increase your experience level by one. An uncursed potion will set your experience points to the minimum for your new experience level, while a blessed potion will set your experience points to a random value from the minimum to the maximum for your new experience level. For example, if your experience level is 2, quaffing an uncursed potion will increase your level to 3 and set your experience points to 40 (the minimum for level 3), while quaffing a blessed potion will set your experience points to a random number from 40 to 79 (80 points is the minimum for level 4).
When you gain a level, you get increased maximum hit points and energy, the amount depending on your level, your race, your role and your constitution. If you already at the maximum level (30), a non-cursed potion of gain level will still increase your HP and Energy.
A cursed potion does not increase your experience level, but does affect your dungeon level (decreasing it by 1), one of NetHack's many puns. Quaffing it causes you to rise through the ceiling. Cursed potions of gain level are most useful for quickly leaving Moloch's Sanctum and other non-teleport levels while on the ascent to the Elemental Planes, and can be used as an escape item in a pinch. However, there are some places they do not work: the endgame levels, Sokoban, the top floors of the Wizard's tower and Vlad's tower, the top floor of the Quest, and Fort Ludios. They also do not work on dungeon level 1, unless you are carrying the Amulet of Yendor. Quaffing the potion in those places will only make NetHack display an "uneasy feeling."
Alchemically, these can be used for the same things as gain energy. This is sometimes useful if you have potions of gain level to spare, however this is not recommended as gaining an extra level of experience is generally a better idea. Especially high-level monsters may be created with this potion as their "miscellaneous item"; upon defeating them, if they have not quaffed it already, you will be able to use it. There is also an alchemy recipe for producing them, and by extension a multiple step alchemy recipe.
If attempting to gain experience levels, blessing potions of gain level before drinking them makes more sense earlier in the game than later, because at high levels the number of experience points needed to level up through normal adventuring is so high that it's rarely done. At higher experience levels, the number of XP between levels is so huge that there is little point in blessing a potion before quaffing, because it's likely the only way you will level up is via potions and wraith corpses.
For example, a 5th level character who drinks an uncursed potion would be set to 6th level at 320 XP, versus 480 with a blessed potion. The difference to reach 7th level is 320 (if uncursed was drunk) vs 160 (if blessed was drunk) which is an attainable goal. On the other hand, above the teens the character is often looking at the difference between 500,000 and 1,000,000 XP, neither of which are likely to be realized through normal adventuring, so blessing the potion makes little difference.
Use by monsters
An intelligent monster will quaff a potion of gain level if it finds one, or if it is generated with one in inventory. If the potion is not cursed, the monster will gain experience, and possibly grow up into a more powerful version (eg. a master lich into an arch-lich). If the more powerful version is genocided, the monster will die. If it is cursed, the monster will rise through the ceiling where possible. (On a level where the cursed potion does not function, NetHack will say the monster "looks uneasy.")
This can be used to identify potions of gain level. If you dip an upgradable object into a potion and receive the message "Hmm! You don't recall dipping that into the potion.", then you will be prompted to name the potion. The potion of polymorph will also change the item, but does not produce this message and also auto identifies.
If you are drinking this potion to yourself gain a level, it is worth blessing it in SLASH'EM. The lower XP requirements for gaining levels mean it is feasible to advance a level by killing enough monsters, unlike in vanilla NetHack where past XL15 or so that becomes completely infeasible.