"Hmm...I guess a little more testosterone couldn't hurt." Sean clicked "Increase" and meant to type in "by a factor of 20%" but instead typed "200%". He clicked "Submit", not noticing his error, and waited for the changes to begin.
Within a few seconds, Sean began to feel itchy, especially around his chest and abs. He looked down, and a thicket of dark hair was beginning to grow in. "Holy shit," he said, his voice at least a few octaves lower than it had been before. He got up, his body starting to bulk up with hard, sinuous muscle, and moved to the mirror.
What he saw astounded him. The stubble on his face was shades darker than it had ever been and coarse as sandpaper. His cock, previously a normal 6" hard, was beginning to expand like a fire hose, passing a foot and still going.
But all Sean could think of was getting off. All the hormones were making him ultra-horny, and as the physical changes intensified, his mind began to revert to a neanderthal state. To match his mental state, his back started to arch forward while his hands doubled in size. Within minutes, he was hunched over, using his fists to help him walk. His brow and jaw, decent enough to start with, were enlarging to mamoth proportions.
Inside, Sean began to lose all semblance of his humanity as the proto-human in him took over. By the time the changes completed, he had lost the capacity for speech and higher thought and started to destroy everything he came in contact with in search of food.
The changes would have to be permament...after all, cavemen can't use computers.