Katie looked confused. "Monster serum. What the heck is a Monster Serum?"
As she reached to take the bottle from Tiff's hand to read the label for herself, Katie's arm seemed to surge forward as the limb expanded both in length and width, as muscle inflated like balloons along it. Claws extended from her fingers. Katie stared open mouth at her uneven hands until her left grew and mutated into an match.
Her brow bulked up which seemed to have the dual effect of causing eyebrows fall out as spines grew out in a mockery of them. The size of these spines increased until The growth of these caused her to cry out in pain, which revealed her teeth were now all razor sharp.
Katie's upper torso seemingly exploded as her shoulder widen and her breasts grew beyond an normal melon even while they were adopting a pointed torpedo shape. The spines along her brow also continued down her back with both an ever increasing size and a wider gap between them. Her skirt and panties were tented by an ever increasing bulge, eventually the fabric was unable to hold it back to reveal a giant penis like rod had emerged from her pussy, of length and width equal to one of her monsterous arms. With a terrible snap her knees bent backward into a degigrade configuration, which combined with the increased mass of her upper body forced her onto fours, her arms now much longer than her legs giving her ape like stance.
The monster that was Katie had a rather bemused expression as it looked at Tiff after it became apparent that the changes had stopped, at least for the moment. It's breathing was a deep rumble as the things blue flat orb like eyes stared at her. It's skin had a dull blue color, it's nipples and penis a dark shade than the rest of her.
Tiff gingerly spoke. "K-katie?"