Lucky Tonic grants one Buffer, which protects one instance of HP loss. Several unblocked hits can still be lethal after that charge is consumed.
Count hits, then subtract Block
An intent with a multiplication sign represents separate hits. Count how many can still reach HP after the first protected hit.
Block and Buffer answer different questions. Block absorbs damage before it becomes HP loss. Buffer matters when an event would actually reduce HP. Read the enemy intent, your Block, your current HP and the number of hits together. A large damage total by itself does not tell you how much Lucky Tonic saves.
For a single heavy hit, one Buffer can be much more valuable than against many small hits. Against a multi-hit turn, Block Potion may cover more damage: its base effect provides 12 Block. Compare the actual turn rather than assuming the rare potion is always the stronger defense.
HP costs and Buffer
The Steam discussion that prompted this question identifies Burns, Withers and self-inflicted HP loss as reasons the protection can disappear earlier than expected. Read your hand and active effects before drinking. A damage-dealing card can spend Buffer before the enemy attacks.
If the same turn includes a self-damaging setup card and a large enemy attack, decide whether the setup is necessary before using the potion. When possible, avoid consuming the charge on a small voluntary loss while leaving the dangerous hit uncovered. Do not play a self-damaging card simply because the attack is still several actions away.
Buffer is also not a revival. It does not restore health or guarantee survival through a defeat condition that is not the protected HP-loss event. A player with one HP still needs to account for every event after the first protected one.
Block before Buffer
Check four things: which hit or status will first reach HP, how much damage remains afterward, whether your own cards spend the charge, and whether another potion or a kill removes more danger. If the remaining damage is lethal, adding Lucky Tonic without changing that sequence does not solve the turn.
For broader potion decisions, see when to use potions. If a supposedly safe turn repeatedly fails because the right defense is unavailable, inspect opening-hand consistency as well as the potion itself.
