poisonous

adjective

poi·​son·​ous ˈpȯiz-nəs 

ˈpȯi-zᵊn-əs

1

: destructive, harmful

2

a

: having the properties or effects of poison

poisonous gas

b

: producing a toxic substance that causes injury or death when absorbed or ingested

poisonous mushrooms

also : venomous

a poisonous spider

3

: spiteful, malicious

poisonous rumors

poisonously adverb

Synonyms of poisonous

• poisoned
• poison
• toxic
• venomous

Examples of poisonous in a Sentence

Chocolate is poisonous to dogs.

This species of spider is extremely poisonous.

Recent Examples on the Web

And in Season 11, before the game even started, castaways hiked 11 miles through the Guatemalan jungle—a 24-hour trek in 100-degree heat that resulted in one collapse, one biceps tear, one vomiting spell, and one puncture wound from a poisonous tree barb. —Kristen Geil, Outside, 24 Feb. 2026

This was always a question of what happens to this friend group when there’s a poisonous dynamic at the center? —Hanna Wickes, Charlotte Observer, 20 Feb. 2026

All parts of delphinium plants are poisonous to humans and pets. —Kim Toscano, Southern Living, 17 Feb. 2026

This was always a question of what happens to this friend group when there’s a poisonous dynamic at the center? —Emily Longeretta, Variety, 17 Feb. 2026

Word History

First Known Use

15th century, in the meaning defined at sense 2

Kids Definition

poisonous

adjective

poi·​son·​ous ˈpȯiz-nəs 

-ᵊn-əs

: containing poison : having or causing an effect of poison

poisonously adverb

Medical Definition

poisonous

adjective

poi·​son·​ous ˈpȯiz-nəs, -ᵊn-əs 

1

: having the properties or effects of poison

poisonous gas

2

: producing a toxic substance that causes injury or death when absorbed or ingested

poisonous frogs and plants

also : venomous