"Your turn, hot stuff," Ben leered, "I want to see what that big cock can do!"
A feral grin stretched across Tony's face, and he positioned himself behind Ben. It looked like Ben had gotten more of the powder; he was still bigger than Tony, even though Tony had been the muscular one before either of them took anything. But now, Tony saw a packet lying out, with a picture of a huge bodybuilder on it. His grin widened, and he rammed his huge cock into Ben's ass.
Ben gasped in pleasure and surprise, as Tony's oversized man-sausage forced its way into him. "Oh god, it's so big!" he moaned, as Tony plowed ever-deeper into his ass.
Tony, for his part, was multitasking. While his cock pounded Ben from behind, he tore open the package, and downed the contents. A burning fire crept through his body, but instead of distracting him, it only made him fuck Ben with greater fury. And Tony grew. His already-large muscles exploded with size, and as his strength increased, his thrusts became even more powerful. Ben was groaning and gasping, as Tony's musclefreak body pounded his ass without mercy. Then, with a thundering roar, Tony came, shooting his load deep into Ben.