To Eric's relief, the scientists left the room (probably to observe him through the two-way mirrors, but he figured that was less weird than them standing there while he changed) and he made himself at home, resting on the bed for a bit. After a while he felt a weird tingling sensation spread through him- the formula must be taking effect.
He undressed, figuring it'd make it more comfortable, prevent his clothes getting damaged and make sure he and the scientists could see what was happening to his body. He sat naked on the side of the bed, looking hazily at himself as the tingling gave way to strange convulsions and contractions.
They didn't hurt, if anything they felt kinda pleasant, but they were very weird. Before Eric's eyes, his figure seemed to suck inwards at certain points, his torso slimmed down while his arm and leg muscles turned more small and lithe and his hands and feet noticeably shrank.
Now his masculine size had been changed, the formula started to make his body shape more feminine. His shoulders were shifted inwards while his hips flared outwards, and his body formed a faint hourglass shape. He noticed his face felt weird, and his eyes screwed shut for a moment; when they refocused, he saw his neck had narrowed and his larynx shrunken away (which in turn made his voice higher-pitched and more womanly).
His eyes traced up him and he saw that his facial structure had changed, his cheeks softer and lips puffier. His facial hair was retracting rapidly, and he felt a shiver through his skin as he realized this was happening almost everywhere. When he touched his skin, it felt much more supple and smooth.
A pant of surprise escaped his mouth as the formula moved onto the last parts of him that still looked male, and it turned into a moan as unfamiliar arousal raced through his body. The young man's nipples became larger, puffier and more sensitive, and as they turned erect he started to massage them, feminine fantasies of having them played with and sucked on forming in his head. He saw his hands move further out from his chest as large soft mounds of flesh formed under these sensitive teats, and he realised these were breasts.
He looked past his mammaries as he felt an intense surge of sensation well up in his penis, and watched as the phallus started to withdraw into his body. To his surprise, it felt weirdly nice to see this. It dawned on him- or rather her- that this formula was affecting his brain, and with it her gender. She felt a sense of anticipation, like after this finished she would be totally a woman.
She leaned in to get a better look at the last part of the change. His balls were tugged up inside her and she felt them reshaping into ovaries and his scrotum sucking up into a vaginal mound. His penis shrank away more and more and she felt the nerve endings concentrate tighter and tighter and form a more and more sensitive nub. It all seemed to culminate in one intense orgasm-like sensation- his penis tucked into a tiny hood and formed her clitoris, a womb formed inside her that connected up with her female reproductive system and his former scrotum formed a vaginal opening which leaked with female juices from her arousal.
Erica sat for a moment, panting and recovering from the intense sensations as they ebbed away slowly. She figured the change must have worked as planned, because she felt entirely female now, to the point even though she knew she'd been male just minutes ago thinking of herself that way felt weird. Her body appeared to have become that way too.
Once she came down from the high, she wondered what to do...