Disclaimer: I don't own Star Trek or any of the characters associated with the series.
The Test
The first time Chekov gets drunk, he's thirteen. It's his second week at Moscow's Academy for the Gifted, and he's planning on spending the night reading through some more advanced physics textbooks he's gotten his hands on. He's bored and lonely and not a little homesick, so when some of the boys from his Cal III class invite him out 'for some fun,' he happily agrees.
He's only had a few sips of alcohol before, but they tell him it's a test of his courage, and, as they're older and cooler and he doesn't want to look like a total wuss, he chugs it down and spends the next two hours, shirtless, trying to explain warp theory to the pretty bartender and singing Russian folk songs at the top of his voice. And when his classmates get bored with that, they herd him over to the physics labs and dare him to transport Professor Basilevsky's doctoral diploma from the wall in her office to the transporter decks in the engineering building without cracking a textbook.
It's not until a few days later, when Chekov proudly informs them of how Professor Basilevsky suddenly nominated him for promotion to the junior class, that he realizes they weren't trying to be friendly.
The fourth time Chekov gets drunk, he's seventeen. It's his second week aboard the Enterprise, and Sulu offers him a celebratory drink for his first successful offworld mission. Before Chekov can say 'bottom's up,' the rest of the bridge crew, plus Scotty and a very pretty blonde nurse, join them in the rec room. Scotty has some Romulan ale, Uhura has a cake and donuts, and Captain Kirk has some loud, absolutely ghastly music that Doctor McCoy complains about for a good five minutes. It's awesome. The fun – for Chekov – however, ends after about an hour, when, in the middle of an explanation about some new cartographical measurements being done near the new Vulcan settlement, he vomits two mugs of ale, one slice of cake, and five chocolate donuts on Commander Spock's pants.
Next thing he knows, Chekov's got one arm over McCoy's shoulder and the other over Kirk's and is being half-dragged, half-carried to the medical bay. Somewhere in the back of his mind, Chekov is aware that he should be embarrassed about having to be hauled off by his superior officers, but his mind is evenly divided between wondering whether the pretty nurse at the party ever wears a regulation minidress and wishing that the captain would stop laughing so much. It's making him nauseous, and he's pretty sure the commander's expression wasn't really that funny. It's not until they're outside the sickbay that he remembers something else.
"In Russia, it's considered a test of courage to see how well you can hold your drink," he says, very aware that his feet are dragging somewhere behind him and that Kirk and McCoy are the only things keeping him upright.
He feels rather than sees the look his captain and doctor give each other.
"Yeah, well, that's a stupid system," says Kirk.
"There's better ways to test for courage than that, kid," says McCoy.
Chekov has a brief moment of feeling wonderful. All is right with the world. "Da!" he says right before they dump him on an empty bed.
He starts humming to himself in Russian when McCoy approaches him with a hypospray.
"You'll thank me in the morning, kid."
As the sedative kicks in and he drifts off, Chekov knows he probably won't thank him, if he even remembers this. He doesn't have to.
It's what friends do.