[Intro]
(I get 'em up!) I'm on one
(I get 'em up!) Fuck it, I'm 'on one
(I get 'em up!) I said I’m on one

from DJ Khaled (Ft. Drake, Lil Wayne & Rick Ross) – I'm On One Lyrics on Rap Genius

Meaning

Usually, “I’m on one” means to be on ecstasy (cf. I’m On One by Gucci Mane)

In this case, however, Drake uses it to mean being under the effects of lean: promethazine and codeine (most likely mixed with sprite). When he says he’s on one, he’s referring to a cup (as seen in the video).

Could be a reference to Eminem:

But in the long run, these drugs are probably going
To catch up sooner or later but fuck it, I’m on one

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