From Nookipedia, the Animal Crossing wiki
Medicine is an item in the Animal Crossing series, appearing from Doubutsu no Mori e+ onward. It can be purchased at Tom Nook's Store for 400 bells (1,200 in Dōbutsu no Mori e+). Medicine can heal sick villagers and cure bee stings. A villager's sickness may last a week and sometimes even longer. If the player provides the ill villager with medicine, after their ailment has passed, they will gift the player with a random item to show their appreciation, as well as provide them with verbal praise upon recieving a dose. Any player can give medicine, though it is the player who contributes the most that recieves the gift.
In Animal Crossing: New Leaf, any villager whom the player is friends with will supply medicine to the player should he or she get stung by a bee.
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