"Hey, Brody? Would you be up for a bit more experimentation?"
Brody looked up, confusion clouding his face. "Why are you asking me?"
"Because I want to know if you're up to it?" Now it was Mark's turn to be confused. "Did you expect me to just force it down your throat?"
"Well, yes." Brody grabbed his shoulders and stared into his eyes. "I may be human, but I have been your dog for far longer. That's just what you do to us. If you want me to do something, I will do it, Mark. No questions asked. A dog is loyal to its master to the end." He knelt in front of Mark.
"So, master. What vial would you have me drink?"
Startled, Mark numbly held out the blue vial again, the male one. Still staring at his master intently with his piercing, electric-blue eyes, Brody snatched it out of his hand and downed it in one quick gulp. He placed it back in the box before the tremors of transformation began to overtake him.
This time, Brody's legs didn't give out, keeping him standing as he doubled over from the pain. There was a great cracking as his bones began to grow and reform, pushing his head closer and closer towards the ceiling as he skyrocketed in height. From 5'10, he grew, quickly outstripping Mark's 6'0 as he straightened, spine popping ominously as he slowed down at an intimidating 7 feet. The seams on his clothes gave way, buttons flying off the horrendously stretched fabric as he finished his growth at an enormous 7'3. Mark looked up at the immense man, hunched over and huffing, the strength of the transformation knocking the wind out of him. But it wasn't done yet.
While Brody was not skinny by any means, the growth spurt had thinned out the muscles he previously had, stretching them across his gigantic body. But that was about to change, as pound after pound of muscle packed onto him, pushing him to the fit, muscular figure he'd had before and then beyond it. Soon, he was breaking the bounds of bodybuilding and reaching into the physically impossible as he swelled grotesquely, every inch of him covered in striated muscle at least as thick as Mark's forearm. And that was just unflexed! As pleasure washed through Brody, he felt a strange compulsion, leading to him crunching into a crab pose, making every vein and artery that crisscrossed his body burst forth as his muscles surged in size.
As he flexed, he felt the pleasure of his growth, a primal pride of the sheer size and power of his new body shooting through him like electricity. It flooded his veins, releasing his alpha pheromones as it passed his glands, and stirred something in his loins. His cock twitched to life, ready for its own bout of transformation.
The 9-inch shaft began to lift upwards, gaining inches as it quickly grew hard, fueled by Brody's flexing and showmanship. It reached its peak at 12 inches, a throbbing steel rod that begged for attention. As Brody shifted one hand to deal with it, he could feel the pulsating beat of his heart with each second. And then, he felt it start to grow. With each second, each moment of unreleased lust, his dick grew longer and longer. 14 inches, 16, 17, each inch multiplying the lust until he was begging, whining for release. The girthy shaft no longer fit one hand, and Brody brought his second hand down just to make its width as he began jacking it off furiously. 19, 20, 21... the cock grew larger and longer, each vein standing on end as the head grew an angry red. As it reached 24 inches, Brody let out a loud whine, growing to a thunderous roar as cum pumped its way down the shaft and came gushing out the end like a hose. Mark could only stare as his dog-turned-muscle-monster painted the kitchen in his thick, white spunk. Brody panted as the last shot spurted out, and his cock finally fell flaccid, an incredible 20 inches of limp man-meat.
He gripped the kitchen counter, before sliding to the floor with a loud thud. He stared in awe at his new body, which outshone Mark's in every way. "So," he said, as he finally caught his breath. "What's next?"