"Poison" is an EDM number, and a modern take on a pop power ballad. It has a moderately fast tempo of 124 beats per minute. "Poison" contains elements of electronic music, combined with a "chewy" pop hook. In an interview for Vice, Ora said that she was baffled by the track being described as EDM-leaning. She felt that it was more passionate and not electronic-sounding at all, and stated "it's more of a rocky, edgy pop record made by a pop star, which is what I am." Aside from the track's crashing production, "Poison" is primarily melodic with Ora soliciting a soaring, stripped-down vocal performance which spans from the note of G₃ to E♭₅.

Set in the key of B-flat major, "Poison" follows a basic chord progression of B♭-B♭/D-E♭_add9, and is drum-heavy in its instrumentation throughout. The song opens with a tender piano progression on which Ora breathlessly emotes, "I could have my beer for breakfast / My sanity for lunch / Trying to get over how bad I want you so much". Sharp percussion and dark string instrumentation begins playing as the chorus builds in intensity. Ora then belts out the song's hook, "I pick my poison and it's you / Nothing could kill me like you do / You're going straight to my head / And I'm heading straight to the edge / I pick my poison and it's you / I pick my poison and it's you ", over prominent synthesizers. Drums later feature most prominently during the middle eight in which Ora sings about her world falling apart.

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