Katherine couldn't stand it any longer. She knew there was only one way that she could get the relief she sought.
Leaning close to Chris, she gently pushed his mouth open, uncapped the vial, and poured it into him. She then held his head back so he would swallow, and put the empty capsule down. The effects did not take long.
Like her, the changes started with the hooves. White fur started to spread upwards, Chris's face extended into a muzzle, and a mane appeared on his back. To finish it all off, a tail sprouted just above his legs. Now Chris was also an anthropomorphic Clydesdale, and a handsome one to boot! Katherine's nose filled with his scent, and her vagina began to drip with lubrication. She didn't care any longer that he was her son. Now, he was a big horse-man.
There was just one final thing left to change. Katherine stared in awe as Chris's penis turned from a tiny little pecker to a 12-inch monster so thick, it looked like a giant worm was eating its way out of his chest like the creature from Alien. A thick layer of skin now covered his cock, and when Kathering gently folded it back, she was shocked to find his penis was practically pulsing with blood, the veins underneath becoming engorged, which only served to further fatten it up. And his tip! His tip was a bright beige, the same color as the nipples on her size-D breasts.
Chris was still asleep, but Katherine could hold herself back no longer. She unfurled her thick tongue and licked her son's tip and shaft until they were wet enough for her to slide on. Chris woke up, but by then he could nothing but grab onto his mother's sides as she bounced up and down on his cock. She could hear him softly neigh in pleasure as his thing went deeper and deeper inside her, until she felt his hands hold her down at the same time that his cock exploded, releasing so much horse jizz that some managed to seep out of her even before he pulled out.
Chris stood there, panting with his still-erect cock hanging in the air. Katherine noticed that the sounds he was making, while unintelligible to human ears, became perfect English in hers.
"Mom, what's happening? Why do I feel so good?"
"Oh, honey." Katherine picked herself up and walked over to her son, putting her hand around his heavy sac. "Look at how fat your balls are. A stud like you probably has gallons of cum still waiting to be milked." She squeezed them lightly for emphasis.
"Mom..." Katherine used her other hand to grab his muzzle. "Not another word. I think it's time you grew up, Chris. I need a man to satisfy me."
Suddenly, Chris had the unmistakable feeling that the floor was getting smaller and his legs were getting longer. Pounds and pounds of muscle coursed through his arms, torso, and thighs. His chest was now as ripped and toned as his mother's.
"Wha-what?", he said in a much deeper voice. "What did you do to me?"
"That potion of yours works in many different ways, son. For me, it made me into the woman I always wanted to be: beautiful, with muscles and nice big jugs. For you, it made you into the man I always wanted. And now, it's time for the finishing touches."
Chris didn't care that his mother wanted to have sex with him. His brain now focused on only one thing: relieving the pressure of his backed-up sperm.
For the next two hours, he banged Katherine with unrelenting fury and passion. He came no less than thirty times, until he nearly passed out from exhaustion after completely emptying his nuts.
Katherine was also exhausted, but she still had the strength left to speak. "Well, that was enjoyable, son. You really put that cock of yours to good use."
Chris said nothing in response. Even words meant nothing to him any longer.
"That being said, we can't stay like this much longer. This-"--she picked up a new vial from the counter--"will turn you back into a human. Plus, it'll make all of this seem like a dream. Every son wants to get it on with the woman who birthed him, right? Then, whenever I need that dick of yours, I'll just turn you back into a mindless, horny stud."
Chris said nothing. He didn't even have the capacity to process what she had just told him.
"Such a good boy. Now find me some clothes."