Sam didn't listen

It was Saturday just before noon when Sam arrived at John's place. He had been ill the last few days, so John had no idea if the pills were actually having an effect on him or not, but he sounded urgent, so of course he allowed him to come over. Worst case if he'd contract any disease, he could just snap it away. John was pretty sure the pills did do the job though, since his powers should make sure of it, and he personally sported a nice 10.5 inches this morning. When Sam came in the first thing he noticed were those baggy pants, and yet a noticeably bulge downstairs. The second thing he noticed was Sam's deeply worried face.

"Thanks." Sam said and gently closed the door behind him before he took a seat on a chair.
"What's wrong?" John wondered. There was clearly something going on. Sam hesitated for a moment, but then started talking.
"Do you remember when you said 'only one per day'?" he asked, John nodded. "So... I didn't listen." A moment of awkward silence passed, where John now finally understood why there was this remarkable bulge in those pants.
"On the first day I just took a second one, though it won't do any harm, and just to kickstart things you know..." Sam continued "Next day I took like... five I think." John raised an eyebrow. "And then I kinda snapped and took a double dose, 5 times, 10 in total." Sam's voice started to crack "And now I am growing, John... I am growing!"
"Wow..." John had to stop himself from laughing and did his best to look like this was a serious issue. "You better stop taking them then."
"I did." Sam interrupted him "I am still growing. I haven't taken any pill for the last 2 days, and yet I still grow over 2 inches a day!" His voice switched to pure desperation "I need help John!"

John did a quick math in his head and figured that his spell that those pills would make him grow 0.2 inches per day actually caused him to grow 0.2 inches per pill. So with 10 pills that was about 2 inches, plus whatever additional growth the growth itself caused. Sure he could just snap him back to normal in an instant, but he was curious.
"So... how big are you now?" he wondered. Sam didn't answer, but started unbuttoning his pants. John felt a bit awkward, but a moment later he stared at a massive schlong, easily as wide as his arms, hanging down past Sam's knees. And he wasn't even hard at all.
"I measured 20.5 inches this morning. But I think I've grown a bit since then."
"Holy..." John had to admit this was some impressive slab of meat.
"But it's worse..." Sam continued "The side-effects are getting to my head. I am so horny all day long... Like everything turns me on, and I can't stop masturbating." John tried to guess the increased levels of testosterone those grapefruit-sized balls would produce, but he could hardly check his magic cheat-sheet to get the exact numbers.
"So like when you think about Sandy, and what she'd do with your cock if she saw it..." John teased him.
"Dude!" Sam tried to hold it back, but lost the fight almost immediately. His eyes rolled back as pure ecstasy from those images in his head was trembling through his body. John watched Sam's dick twitch, then rapidly grow fully erect, till he sported a massive tower of man-meat, that reached up to his chest.
"Sorry... I can't hold it back anymore." Sam panted. John could clearly see how he was ready to go for it, just the fact that John was around kept him from working his member. But those last attempts of restraint could break at any moment. John knew it would take him just a second to fix this, but should he do it? Or should he first watch where this was going. And that small evil devil on his shoulder told him to instead just make it even worse for poor Sam.