painkiller vs medication

painkiller and medication both are nouns.

Word	Noun	Adjective	Verb	Adverb
painkiller	Yes	No	No	No
medication	Yes	No	No	No

As nouns, medication is a hypernym of painkiller; that is, medication is a word with a broader meaning than painkiller:

painkiller: a medicine used to relieve pain
medication: (medicine) something that treats or prevents or alleviates the symptoms of disease

Other hypernyms of painkiller include medicament, medicinal drug, medicine.

painkiller (noun)	medication (noun)
a medicine used to relieve pain	the act of treating with medicines or remedies
	(medicine) something that treats or prevents or alleviates the symptoms of disease