Columbia subject headings:
Harvard subject headings:
Analgesia
Drug addiction--Pathophysiology.
Drug addiction--Physiological aspects.
Endorphins--Physiological effect.
Endorphins--Receptors
Pain--Physiological aspects
Princeton subject headings:
Analgesia.
Drug addiction--Physiological aspects.
Endorphins--Physiological effect.
Endorphins--Receptors.
Pain--Physiological aspects.