But that tab
Is light blue, she take it straight to the head

from Kendrick Lamar – A.D.H.D Lyrics on Rap Genius

Light blue is the color of a small adderall pill, which is a perscription drug for ADHD but young people often also use it as a party drug to stay up and make their alcohol tolerence stronger. This may also refer to lortabs, prescription painkillers that are often abused at parties. .

Except she doesn’t take it the regular way, she is snorting it (“straight to her head”)

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