Poisonous
Toxic, Venomous
noun, な adjective, の adjective
When something has poison it is poisonous and toxic.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
ガラガラヘビって有毒なんですか？
Are rattle snakes venomous?
これ、タバコより有毒なんだよ。知らないの？
This is more toxic than tobacco, you know? Don't you know?
どうやら有毒なガスが発生しているようです。
It seems that toxic gas is being emitted.