One thing that this brought to my mind is the potion that Regina drank to become infertile, which seems possibly relevant in light of Robin’s death and the Evil Queen’s return.
There have been a number of less-than-happy potions, though, like the one that Rumple drank so he couldn’t save Neal from his fate, or the one that Snow drank to forget Charming when she thought he had to marry Abigail or die.
But yeah, I’m on team this-has-to-do-with-Jekyl-Hyde.
Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing