My serotonin's gone a while to go
Girl, it's gone cause of you
It's been gone cause of you

from The Weeknd – Heaven or Las Vegas Lyrics on Rap Genius

Meaning

They’ve been taking ecstasy together, which activates the serotonin receptors in the brain, releasing feelings of intense pleasure. However, after heavy ecstasy use, serotonin levels can be permanently lowered. Clearly, he and this girl have been rolling heavy and for a while..

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