It happens as soon as he steps out of the cab that brought them to Barts. A wave of dizziness rushes over him, and he finds himself struggling for breath – he's vaguely aware that John is talking to him, but he can't really understand a word of what he's saying.
He's sweating and trembling, his own heartbeat pulsing in his ears. He always hates it when he's sick, when his body refuses to obey his mind; now his mind seems to have gone haywire as well, he can't form any coherent thought and the very idea only makes him panic even further.
His brain is all he has, all he is; he can't afford losing control, can't afford losing his mind.
He wants to scream, and yet no sound comes out of his clogged throat. After that everything is a blur; when he starts breathing again he's lying on his bed in Baker Street, though he has no recollection of how he got there.
"It's alright, Sherlock," John says as soon as he tries to get up, his hand firmly on his shoulder. "Lie down."
"What happened?" he asks, his own voice too shaky to his liking. "Am I going insane?"
John shakes his head. "You had a little panic attack, that's all. Nothing to worry about."
However, he can see that his friend is worried, and that does nothing to quell his fears. This can't happen again, he needs absolute control over himself – body, mind, and emotions alike.
He's always known how dangerous emotions can be, always tried to avoid them; he wasn't really expecting to be overwhelmed at the mere sight of the place where he had to take the hardest decision of his entire life, two years ago now.
Up on that rooftop, looking down at John right before taking the final step.
"Breathe, Sherlock," his friend urges him, and it's only then that he realizes he's hyperventilating again. "In, and out. Slowly."
He does, focusing on the familiar voice that effectively soothes him. As much as he loathes showing his vulnerabilities, he can't help closing his fingers around John's wrist; he needs the contact to reassure himself that his friend is still alive, and he's not going to disappear anytime soon.
"Won't tell anybody, I promise," he mutters, closing his eyes and relaxing against the pillow.
He knows that John is rolling his eyes now, and that's enough to bring a smile to his lips at last.