It would have if not for a stray hair falling from Harry's head

Unbeknownst to Harry, as he was pouring the potion into a cup, a single strand of hair had fallen from his head and drifted down perfectly into the cup. The potion reacted to the additional ingredient and briefly bubbled before the tint of the potion shifted towards a more vermilion colour. Lost in his fantasies of bending over Ginny and fucking her senseless, Harry did not notice the chance as he drained the contents of the cup down his throat.

Ba-dump!

"Ah!" Harry cried out in pain as a sudden squeezing sensation in his chest appeared and stole away his breath. The cup in his hands fell from suddenly nerveless fingers as he soon dropped to the floor himself, writhing in pain.

'What's happening?! Why does it hurt...so much?!' thought Harry frantically as he felt his insides start to twist and writhe inside of him as if they were fighting to burst out of his skin. The pain was excruciating and Harry found himself unable to physically call out for help. He saw his hand contort and compress in on itself, seeming to shrink and take on a more slender form but mixed with the awful sound of bones breaking from all over his body.

As the pain reached a crescendo, Harry was blearily reminded of the time he saw Lupin transform into a werewolf and how agonizing the transformation had seemed. He was thankful when the agony of his transformation finally reached levels where he passed out.