Well, in these stories, the fantasy medicine is typically associated with some sort of otherworldly power. It might be related to a specific magical creature or a long - lost spell. Also, the change that occurs due to the weight gain is not just physical. There's an emotional and social aspect too. For example, the character who gains weight might gain confidence and acceptance in their community. And most of the time, there's a bit of mystery surrounding the medicine, like its ingredients are either very rare or unknown to the common folk.
One common element is the setting. These stories are often set in a world full of magic, like a medieval - style fantasy land or a magical forest. The fantasy medicine is usually presented as a solution to a problem, such as a character being too frail to fight or being shunned because of their appearance. There is also the idea of the medicine having a unique origin, perhaps created by an ancient race or a powerful mage. And the effects of the weight gain are not always immediate but rather unfold over time, adding to the sense of wonder and magic in the story.
Magic is a common element. The medicine is often magical in nature, like a potion or an elixir. Another common thing is the transformation. The characters usually go from being underweight or weak to strong and healthy with a fuller figure. And there's often a sense of discovery, where the character stumbles upon the medicine by chance, like in a hidden cave or an old wizard's lair.