Mark looked over the vials again and was intrigued to find one that had a letter 'Y' on it. Curious about what this would do he shrugged his massive shoulders and drank the whole thing. For a while nothing seemed to happen. He was standing there, but noticed no real changes. His face felt a bit scratchy, like his facial hair was shifting, but nothing beyond that. Little did he know that he was now no longer 25, but 20. Then the itchiness stopped and, feeling disappointed, Mark drank the now refilled vial again.
This time he noticed the changes much more. As he slipped down to 18 he felt himself getting shorter, and his muscles were shrinking too, though they seemed to be doing so proportionally. His hair was retracting a bit too. By the time the regression stopped he was about 6 feet tall, down from 6'6'', and still as massively built as before, and boasting a similarly huge package, but his hair had gone down to about the level it had been before the second dose of male.
"I wonder..." he muttered, his voice quite deep for a 15 year old.
Downing another dose, Mark noticed changes much more immediately. He continued shrinking and losing chest hair, his facial hair almost gone, and his face and body gaining a more youthful, childish look. It seemed the vial took away five years at a time, because when his regression stopped he knew he was now 10 years old.
For a 10 year old though, he was massive. He stood 5'2'', huge for a 10 year old. His muscle mass was still the same too, and when he flexed he could see huge biceps spring up, and his eight pack still showed. He felt his ass to find it was still round and firm, and his manhood was about six inches long flaccid. As he looked over his muscles it began to erect again, showing a good eight inches. There was even some beginnings of pubic and chest hair, the latter around his navel.
Rushing to the bathroom Mark saw his face was clearly a child's, but there was a strength to his jaw and brow still. His skin was youthful and smooth, but there wasn't an ounce of fat on him. "Woah," he muttered, his voice boyish, but quite deep for his age. "I guess I need to find some clothes."