"You know what we need to do now," said Ms. Heather. Justin was scared to ask, but he saw her look at him with a predatory gaze. "Let's see how this formula affects you Justin."
He didn't feel much like he had a choice in the matter. The look in her eyes said she wouldn't take no for an answer, and her muscles meant she could pretty much force him to do anything she wished. Grabbing the last vial left, the yellow drink, she walked over to Justin and handed him the formula. He gulped nervously and he could feel his heart hammering in his chest, but he swallowed it all the same.
He felt the warmth spreading through his body. His arms and legs began to swell, feeling full and pumped. His shirt grew tight as his pecs swelled, and soon he saw small tears in the fabric. As his lower body filled out he could feel his pants straining at the seams. They were getting uncomfortably tight when all of a sudden he felt them burst along the legs, showing off thick, well muscled thighs. Then he felt his butt tear through the back of his jeans and blushed, realizing he was now in his underwear. Ms. Heather could now clearly see what a bulge Justin had. He was big, and clearly sporting a massive erection. She couldn't blame him. After all, she had nearly orgasmed over her own growth. She knew how exciting it was to grow massive.
Justin's growth began to slow now. His shirt had somehow survived, though it was practically painted on, showing huge pecs and defined abs through the fabric, and even his erect nipples. He looked Miss Heather in the eye, both of them smiling at each other. "Tear it off," she said. Not a suggestion but a command. Justin reached up and grabbed his shirt in one hand, ripping it off with no effort.
He flexed his new muscles, breathing heavily at the sight of them. He looked like a Greek statue. Ms. Heather was clearly excited herself. When Justin finally looked away from his new biceps, he was surprised to find her with her fingers in her pussy, shamelessly masturbating right in front of him. He slowly lowered his hand, stopping at the waistband of his underwear, tugging at it, but not revealing anything yet. Ms. Heather could see that he was teasing her, and masturbated even more furiously at the display. Just as she was getting close to orgasm Justin grabbed his underwear and tore them off completely, revealing a gorgeous hard cock. Ms. Heather nearly drooled at the sight, and was hit with a massive orgasm, screaming squirting all over herself.
Coming out of her orgasmic bliss Ms. Heather looked back at Justin. They could clearly see the lust in each other's eyes, and knew what was coming next. The two crashed into each other. Justin fell backwards as Ms. Heather was still much larger than him. As she positioned herself above Justin's dick, he felt liquid hit his thighs. She was literally soaking wet.
She dropped herself onto Justin, both of them moaning as he slipped all the way inside her in one motion. Ms. Heather put her hands on Justin's meaty chest and started moving. Justin reached up to feel her, groping at her massive triceps and shoulders. He moved to her abs and could feel them contracting as she rode him. He finally settled on her chest. She moaned when he touched her boobs, and soon started flexing her pecs as he groped her, making her chest bounce even more and showing off the hard muscles underneath. Ms. Heather moaned and squeezed her powerful hands on Justin's chest, digging her nails in and causing him to gasp from the pain and pleasure it caused. Then Ms. Heather grabbed Justin and pulled him up to put his face in her cleavage. Her powerful arms had him trapped there. He could feel how soft her breasts were, and how solid the pecs just below them felt.
Ms. Heather's moans started getting louder and she began moving faster as she felt her climax approaching. She let out a thunderous roar as she orgamsed, the walls of her pussy clamping down forcefully on Justin. The squeeze he felt from Ms. Heather spurred his own orgasm, and he moaned into her chest as he shot hot ropes of cum into her.
Ms. Heather got off Justin and lay down on the floor next to him, basking in orgasmic bliss. "This is a marvelous invention Justin. Top marks," she said, then leaned over and kissed him. Just as they finished their kiss she started breathing heavily. Her body was pumped to the extreme. Her muscles were growing again. She nearly screamed as she flexed her new inhumanly large biceps. "What was that?" she asked excitedly. "You didn't give me the chance to explain all the parts of the formula," Justin replied. "When men take it, their cum causes women to grow even larger." Ms. Heather looked down at Justin with newfound vigor and lust.