Saving every potion for a boss can cost more than it preserves. An early potion can prevent the damage that would force a later rest, keep an elite route available, or stop a dangerous enemy from taking another turn. The useful comparison is the current result with and without it.
Immediate potion value
Fire Potion deals 20 damage. If that finishes an attacking enemy, it can save more health than a defensive potion because the enemy’s attack never happens. If it leaves the target alive and the same attack still lands, the immediate benefit is smaller.
Block Potion supplies 12 Block. Compare that with uncovered incoming damage rather than the full intent. If your cards already block everything, the potion is unnecessary for that turn. If it prevents a large loss before another forced fight, spending it can keep several later choices open.
Energy Potion supplies two energy. Count the cards it lets you play: an extra strong attack, a needed Block card, or a setup effect that prevents future damage. Extra energy with no useful cards in hand does not repair the draw. Swift Potion draws three cards, but those cards still need to be playable with the resources left.
Match the duration to the fight
Strength Potion grants two Strength for the combat. Its benefit can accumulate across many attacks, especially multi-hit cards. Flex Potion gives five Strength that is lost at turn end; use it on a turn where enough attacks can exploit the temporary bonus.
These are different jobs. Drinking Flex on a setup-only turn wastes its damage opportunity. Holding ordinary Strength until the final attack misses earlier benefits. A potion that repairs one bad turn can be better than a longer-lasting effect when that one turn would kill you.
Potions before an elite
Before choosing an elite, assign each carried potion a plausible use. A damage potion can shorten the opening; a defensive potion can cover the expensive setup turn. That makes a harder route more reasonable only if the rest of the deck can handle the fight.
Do not count on finding a replacement immediately after spending one. Reward availability is uncertain. Equally, a full belt is not a reason to keep an unsuitable potion forever while declining better rewards. At the next potion reward, compare what you carry against the coming encounters.
If you repeatedly die with potions unused, review the first turn where one would have changed an attack, kill or draw outcome. The mistake may have happened well before the lethal turn. For one particularly misleading protection effect, read Lucky Tonic’s limits; for deciding which fights to prepare for, use route planning.