Mary swallowed the whole vial in one shot.
She didn't feel anything at first, but then after a few seconds, she felt a burning sensation from her stomach.
"Oh god, I'm going to be sick!", she thought.
Mary ran down the hallway to her bathroom. When she turned the light on and looked in the mirror, she could see that she was sweating rapidly. Then she felt her body starting to itch rapidly as well.
"I didn't think it would be this painful," she thought as she took off all her clothes to see if it would stop the itching. Within about 30 seconds, her watch, t-shirt, bra, socks, pants and underwear were on the floor in the hallway and she stood naked in the bathroom but still felt her whole body in pain.
Then as she looked down at her feet, she could see her skin starting to change colour from white to red. Then one by one, other changes happened. Her toenails became long and black, her pubic hair (which she had only shaved off last night) grew to be thick and black, her boobs and butt became bigger and her waist became smaller giving her body an "hourglass" shape, her nipples turned black, her fingernails became long and black, her ears became pointy and "elf-like", her blue eyes became red with the whites in her eyes turning yellow, then finally her blonde hair changed colours to become black.
Mary looked in the mirror and was just beginning to process what had happened to her body, when she felt the pain come back again.
"Dammit, what now?"
She looked behind her and saw a devil-like tail growing out from her tailbone and becoming about as long as her legs, two bat-like wings came out from about the middle of her back and were about as long as her arms. Last but not least, a pair of devil horns grew from the side of her head and were about three or four inches long.
Her transformation was finally complete. Mary had transformed herself from a regular woman into a succubus. She wasn't exactly sure what to do now though. Would she go to her bedroom and try to pleasure herself in her new form? Or would she go out on the town and she what her new look could do?