Season 1, Episode 6 "Vitamin D"

Vitamin D is not actually a kind of medicine, it refers to Will’s plan trying to get glee club to step up their game. Unfortunately, glee thinks sectionals are going to be a breeze and so they aren't putting in any effort. Why does the club think the competition is going to be a cakewalk? They're competing against a school for the deaf and an all-girls halfway house. My spidey-senses say those clubs are going to be exceedingly excellent, despite what glee thinks.

Finn Hudson is exhausted by his extra-curricular activities, so Terri gives him pseudoephedrine tablets, which Finn shares with the rest of the males in the glee club. The effects of the tablets enhance their performance, and they give an energetic performance in the mash-up competition.

Kurt tells the girls the secret behind the boys' performance, they, too, request the tablets from Terri, and give a high-spirited performance. Finn and Rachel feel guilty for cheating, however, and agree to nullify the competition. When Principal Figgins learns what has happened, he fires Terri and, angry with Will, appoints Sue as co-director of the glee club.