"The Novel of the White Powder" by Arthur Machen

Machen was a pioneer of supernatural, fantasy, and weird fiction. H.P. Lovecraft and Stephen King both praise his work as “maybe the best horror stories in the English language.” 

This story is about a young woman’s brother who takes a medicine that sat on the apothecary’s shelf too long and mutated into something… not good for humans. Perhaps we should all be careful about the expired meds we have in our cabinets...