Take it in vain,

from Kendrick Lamar (Ft. RZA) – Ronald Reagan Era Lyrics on Rap Genius

Meaning

Refers to taking drugs through intravenous injection, and also to not thinking about something before doing it. Example: taking the Lord’s name in vain. This successfully ties in with both the Vicodin mention afterward and the Daddy Kane brag prior

Also possibly implying that when Vicodin couldn’t ease the pain (physical, emotional, or otherwise) they turned to heroin as a way to escape it.

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