"Woah, that felt even better than last time!" Andrew exclaimed, entirely oblivious to his deeper voice. "Do you have any more of that?"
Dazed, Mark shook his head. "That was the last serving." At the crestfallen expression, he couldn't help but add, "But I have more chicken in the fridge, I'll cook you more if you want?"
"Yes, please! Oh, and do you still have that exercise machine in your basement?" Mark nodded. "Cool! Can I use it? I wanna see what real weight does to these things."
Before Mark could warn him against the side effects, Andrew had already disappeared down the stairs and into his basement. Mark sighed, running a hand through his hair. He wasn't sure what it was, but every time he saw those massive, manly muscles, he just melted before the sheer dominance of his younger cousin. He was helpless. Resigned, he went to cook some more chicken, when an idea hit him. What if he added a vial to the chicken, and cooked it in it? It would be the perfect delivery method, to reverse Andrew's changes.
...or to make them worse.
Mark gasped and shook his head. No! That wasn't what he wanted! He wanted his cute, 11-year-old cousin back, not this weird muscle-bound freak. Even if it was kind of sexy...
Fuck it.
Mark grabbed the chicken out of the fridge and threw it into a pan before grabbing a vial. He looked at the label - 'Release the Beast' - and dumped it all in. Add a little 'Male' vial to the mix, and a slight hint more 'Muscle' for seasoning, and the chicken was ready. He plated up the chicken breast and walked downstairs to find his cousin.
Andrew was lifting a barbell, the one that Mark had never even touched, above his head. In the few minutes Mark had taken to cook, it seemed that Andrew had put on even more muscle, lifting the 80kg bar with ease. It dropped to the ground, and he picked up the 50kg dumbbells, and Mark watched in awe as every bicep curl bloated them to even greater heights. He must have made an audible sound, because Andrew turned around, discarding the dumbbells and rushing over.
He didn't even ask before stuffing his face with the chicken, not bothering to use anything but his hands to scoff the potion-infused chicken down his throat. "Mm, thanks man, that was great!"
Mark put the plate down and warily backed away. Andrew eyed him strangely. "What? Is something wrong?"
Suddenly, he doubled over, groaning as his 6'5 height exploded upwards, to 8 feet and then 9, stopping at a huge 10 feet tall. His naked body bloated with muscle, going beyond the ranges of a bodybuilder and into the impossible. He even began to develop muscles that weren't even natural, with two sets of biceps and triceps, and huge lats that tapered down to his much smaller (though still immense, by any consideration) waist. Instead of a six-pack, more and more slabs of muscle piled on until his chest was a huge wall of 18 abdominals, heaving obliques creating a picture frame around them.
Mark knew when the rest of the 'Male' vial had fully taken effect, due to the pungent odour that Andrew suddenly excreted. His stubble and short body hair burst out, creating thick forests of manly fur as his blond hair became ebony, leaving him with a rugged, wild smoulder. His sweat, which glistened thick on his muscles, became musky, almost hypnotic, dominating all senses and leaving Mark rapt in Andrew's presence. Then, the hulking man roared.
It burst from his throat, loud and wild, as his voice dropped several octaves to a deep, bone-shaking growl. In his mouth, his teeth grew sharp as daggers, and from his huge hands, dark claws grew as long as knives. Black fur ran down his back, connecting to the mane on his head. Somehow, he seemed to grow even more muscular and monstrous, dropping to all fours as his eyes glowed red. Still, Mark could see within the burning embers of his gaze a far too human intelligence, paired with all the savage nature of a beast and the pure strength of his body. The man-beast dropped his head, stalking over to Mark and backing him against the wall.
"How strange, I don't remember the muscle vial doing this to me," he growled, a twisted grin on his bestial visage. "Perhaps you could explain to me, big cousin, how I got this way?"
"P-please don't hurt me!" Mark stammered. Andrew laughed, like the howl of a wolf.
"Hurt you?" His gleaming red eyes glowed menacingly as he pinned Mark down with one hand. "I wouldn't hurt you. You made me stronger, Mark, you gave me power, so I'm going to reward you."
His other hand reached down. Mark followed his gaze to reach the inhuman cock that was beginning to stir at Andrew's groin. Two feet long, soft, with balls bigger than his head and a watermelon-sized knot at its base. The python slithered up his thighs and poked at his jeans, and Mark realised with a twinge of terror that even Andrew's cock was bloated with muscle, making it completely prehensile and under Andrew's complete control - and able to literally fuck the brains out of a person. Pre-cum spilt onto his pants, revealing his own, pitiful bulge.
"If it will hurt, that's down to you."