Antiseptic
Noun
ぼうふざい
古い防腐剤はどこに捨てればいいですか。
Where should I throw away old antiseptics?
A prevent rot dose is a dose of something you put on your cut or wound that prevents it from rotting (or you put it on something else to prevent it from rotting). This is an antiseptic or preservative.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.