"Okay, lets do this!", says Mary and starts drinking the blood. "Hm, doesn't taste as bad as I thought it would!"
More and more of the red stuff flows down her throat. After 5 minutes, she stops, exhausted from drinking so much.
"Now it is time to wait...", thought Mary to herself. And indeed, quickly after she stopped drinking she noticed herself growing. "Its working! Oh my god, I cannot believe this!" She grows and grows, inch by inch. After a few minutes she finally reached the height of 1 feet. "This isn't much, but its a start to get out of this mess. And god, this growing felt so good! Okay, lets get back to it. I need to reach my old height after all." And thus she starts drinking blood again. After another 5 minutes she already reached the height of 2 feet.
"Oh this feels soooo goood....", said Mary, not noticing the other changes occuring with her body. Her hair darkened and some of it even fell off. Her skin darkened as well, some of it even hardening a little bit. Her eyes changed colour from brown to black. Mary, however, didn't really notice yet and just wanted to drink more blood...