Beta-endorphin
Beta-endorphin was an opioid peptide neurotransmitter found in the neurons of both the central and peripheral nervous system.

In 2375, The Doctor found that Harry Kim's beta-endorphins were abnormally elevated following  with Derran Tal, explaining why Kim felt wonderful. ()

In 2377, The Doctor told Icheb, who was laboring under a misapprehension involving B'Elanna Torres, that increased beta-endorphin production was a physiological response indicative of one person having romantic feelings for another. ()

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Biochemical compounds