Fairies are known to be tricky, and the forest is so easy for them to hide in. John had no choice, he took the fairy vial and drank it.
The first think John noticed was himself getting smaller. His clothes became even baggier as he shrank to around 6 inches in height. That was to be expected, but not what came next.
His hair began to grow to his shoulders, slightly budding breasts developed on his chest, his waist pinched in slightly and his boyhood shrank into a small crevice between her legs. John's surprise at this unexpected change quickly waned as he felt a pressure on his back. She felt her wings push forth and grow into a beautiful, clear, colorful, butterfly shape. Did this mean that fairies are all girls?
John didn't have time to consider it as there were more important matters to take care of. She couldn't go out naked, though, so she chanced a spell to shrink her clothing to a fitting size. She tried her new wings out, lifting off the ground easily. Though the sensation was new and wondrous, John realized she had to focus on the task at hand, finding her real sister and bringing her home.