painkiller vs pain pill

painkiller and pain pill both are nouns.

Word	Noun	Adjective	Verb	Adverb
painkiller	Yes	No	No	No
pain pill	Yes	No	No	No

As nouns, painkiller and pain pill are synonyms defined as:

Other synonyms of painkiller include analgesic, anodyne.

painkiller (noun)	pain pill (noun)
a medicine used to relieve pain	a medicine used to relieve pain