Sherlock steps toward the stairs before John starts to fall. He can see it shortly before it happens in the way John's foot comes down. Not far enough forward. Heel will slip. Right foot will bend, slide, and then fall. Left leg will buckle from the weight. Hands will find nothing to grasp. And down John will go.

It plays out exactly as he expects, but Sherlock stops dead in his tracks by the look John gives him over his shoulder. The expression itself would be funny in any other situation, but right now it's just hard to look at. He's seen drunks before and he's dealt with addicts on a binge, but he'd never thought much about it. Seeing his only friend go through these sloppy motions and lose himself in the process is nauseating. He's not disgusted by John, but he's starting to feel revolted by some of his decisions. It's not his place or responsibility to be a caregiver.

"I can deduce my way through quite a lot. Right now, for instance, I can deduce that you're going to require assistance if you want any hope of getting to your bedroom. Once there, you'll likely pass out and wake up in two hours time in a puddle of your own sick," Sherlock tells him, avoiding addressing his friend's concern for his safety. Feeling like he's given a fair warning, Sherlock takes the next few steps and bends down to slip his hands under John's arms without waiting to be invited to do so.