enkephalin
noun
en·keph·a·lin in-ˈke-fə-lən 
-(ˌ)lin,
en-
: either of two pentapeptides with opiate and analgesic activity that occur naturally especially in the brain and have a marked affinity for opiate receptors compare endorphin
Example Sentences
Recent Examples on the Web During that time, your body tends to produce less endorphins and enkephalins, which are natural pain-killers, than at any other time of day, according to the National Headache Foundation. 
Korin Miller, SELF, 24 Aug. 2017
Korin Miller, SELF, 24 Aug. 2017 Word History
Etymology
enkephal- (alteration of encephal-) + -in
First Known Use
1975, in the meaning defined above
Medical Definition
enkephalin
noun
en·keph·a·lin in-ˈkef-ə-lən, -(ˌ)lin
: either of two pentapeptide endorphins:
a
: leucine-enkephalin
b
: methionine-enkephalin