The vial's effects were still causing Mary to feel drowsy, just like the first time she had tried the cow vial. Even though it was only late in the morning, she yawned widely and stretched out, feeling her udder wobble as she did so.
She decided she'd better take a quick nap. Mary got back into bed and soon found herself drifting off to sleep.
She was grazing in a meadow, standing on all fours with her head low to the ground. Around her she could hear the grass rustling in the wind and insects buzzing around from flower to flower. The grass tasted pleasant and crisp in her mouth, and the sun was warm.
She opened her eyes and felt the sun beating directly on her. It was around six in the evening, and the setting sun was peering in through her window. She yawned and sat up. Her udder was still there, just as before, and in this sitting position it felt a little squished. She stood up out of bed and the feeling subsided. However, she felt something else on the back of her legs. Turning around, her suspicions were confirmed.
A black and white cow tail had once again formed from her backside while she slept. It dangled a couple of feet and ended at a cuff with the same color hair as on her own head. Why were cow features still growing on her body?
Mary went back to the vial box and examined it carefully for additional instructions. On the very bottom of the box, she saw small words inscribed on a white label.
"The Vials can transform you all or partway into an animal of your choice. However, once in a complete animal form do not take the vial for that animal again. Unpredictable reactions may result. If this occurs, wait until changes have stopped for a full 48 hours before using any more vials."
So that explained it. She was already fully human when she took the female human vial, so her genes had become unstable. Still--she looked over her body once more--having a cow udder and tail didn't seem bad at all. Mary's only concern at this point was losing so much time from work. She had already taken two days off already. Perhaps there would be a way to hide the changes if she dressed appropriately?