I'm the drug in your veins, just fight through the pain

from The Weeknd – What You Need Lyrics on Rap Genius

Meaning

She loves having sex with so much that she fiends for him, he’s her drug (speaking of which “And I'ma give you girl, oh what you fiend” is the correct lyric for the line before this one which makes this line make more sense)

“Fight through the pain” refers to the pains of withdrawal, the various biological difficulties caused by going too long without an addictive substance after prolonged exposure.

He’s become her drug, the addiction running through her veins. Without him, she’s in pain.

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