pain pill vs painkiller

pain pill and painkiller both are nouns.

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

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

Other synonyms of pain pill include analgesic, anodyne.

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