Permanent Potions (5e Variant Rule)
From D&D Wiki
Some stories I've seen have potions take on different levels of importance, and I've seen plenty meant to have permanent effects. Where's that in D&D you ask? Why, you oddly specific question asker, it's right here!
Permanent Potions have different properties then their normal counterparts, often having completely different ingredients and rarities entirely, sometimes with even different effects! These differences in effects are usually based on how the mechanics of the game work, preventing the potions from being too powerful as their one-time use gives the drinker a permanent extra ability.
Some potions may be considered permanent, as though a 00 (100%) was rolled on the Potion Miscibility table (Dungeon Master's Guide p. 140).
Rarity. As a general rule of thumb, a homebrew potion can be given a permanent version by increasing its rarity two steps. If this puts it above Legendary, consider if it should exist at all, and if you still want it, consider using the Unique rarity homebrew ruleset.
Cost. Unlike other consumable magic items, a permanent potion's creation cost and sale value should not be halved.
Mixing Potions. If you are under the effect of a permanent potion, then drinking a normal non-permanent potion does not trigger a roll on the Potion Miscibility table. However, drinking further permanent potions does trigger a roll on the Potion Miscibility table.
Finally, keep in mind that not every potion has a permanent effect that is desirable, such as the Permanent Potion of Gaseous Form (5e Equipment). The process for removing the effect of a permanent potion is at the DMs discretion, and may involve dispel magic, greater restoration, remove curse or a ritual that must be discovered as part of a quest. It might require a different custom potion of the DM's own design to cure, or some ancient magic that's long forgotten on the world. Generally, the quest should be slightly more difficult then that which acquired the potion, if the party sought out the potion at all.
Below are all the Permanent Potions versions of each potion in the DM Guide. The process to make these potions would likely be fairly arduous, perhaps involving a quest to find the ingredients (And/or someone who can even make it), before being found. The likeliness of these just being found somewhere is extremely low, or at least SHOULD be extremely low. Keep in mind that other types of Permanent Potions with Homebrew effects can be made, quite easily if you know what you're doing. If you do so, take a look at the list below, and see what I did to keep things relatively balanced (Such as how sometimes potion effects that give spell effects normally instead give you the ability to cast these spells, rather then them always being active. I could have done that for the Permanent Potion of Invisibility (5e Equipment) too, but there was NO way I was nerfing that ridiculous possibility.
DM Guide Default Potions
Permanent Potion of Giant Strength (5e Equipment) Special thanks to Magierpichu for helping me with the table!
Permanent Potion of Health (5e Equipment) Because a Permanent Potion of Healing wouldn't really make sense to say.
Permanent Potion of Longevity (5e Equipment) Which is akin to potions that cause immortality, and may be called by such an appropriate name.
Permanent Potion of Water Breathing (5e Equipment) My pick for the most boring, yet most practical one.