A potion is used instantly and cannot raise the player's HP above 50. This means that if a player is at 50 health and buys a health potion, the purchase will have no beneficial effect. If the player is using the website, this will waste 25 gold points (this gold can be restored by using Fix Character Values), but if they are using the iOS or Android app, they will be prevented from making the purchase. This also means that, unlike many other RPGs, health potions cannot be bought to stock up for a later time.
There is no limit on the number of health potions that can be used per day, provided the player has enough gold to buy the potions.