Mary took up the cow vial. "If my regression is because I took too much of the female human vial, then if I take enough of the cow vial to match it, maybe it will balance things out."
"I'm not sure," Rob said. "It might just make things worse."
"What have we got to lose? I happen to like cows," Mary winked. "But get those bowls ready again, just in case."
Rob looked queasy. He wasn't ready to start milking a twenty-gallon udder for the second time that day. He watched Mary swallow several sips of the vial. She wobbled for a second, but didn't faint.
"I think I'm getting used to this," she smiled. She picked up another slice of pizza and continued eating. They both remained silent, waiting for any changes to Mary's body. When the pizza was nearly gone, Mary suddenly lurched. Rob watched her robe expand slightly, and the visible parts of her body thicken. She looked like she had instantly gained twenty pounds.
Mary moaned, quickly fumbling to untie her robe. Her udder sprang out, and to Rob's horror was inflating once more. Fortunately, however, it wasn't getting as large as it had before. He could also see Mary's exposed bra, and blushed, but he couldn't bring himself to look away. White and black hair was growing over her entire body. Within seconds, she had a thick coat of spotted fur. Her nose flattened and got a little wider, as did her face and neck. Her horns grew a couple more inches. As she felt her face with her hands, Rob noticed they were getting hard and black. Her fingers melted together completely, morphing into solid cow hooves. Her thumbs shrank into little stubs. He looked down at her feet, seeing two more hooves and oddly-jointed legs.
"Ooooh," Mary moaned, rubbing her udder with her hooves. "Rooob, it feels sooooo goooood..."
Hearing Mary's half-mooed reply frightened him more than anything. He decided he'd better change her back as soon as possible. It looked like her changes were over. She was still standing perfectly well on two legs, and her breasts hadn't seemed to disappear, so that meant there was still a good deal of humanity left in her DNA. Maybe only a quarter of the female human vial was all she needed. He looked through the vials for it, but was prodded on the back by Mary's hoof.
"Noooo," Mary tried to say. "Dooon't. Roooob, trrry ooone foooor yooooursellllf!"