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

Carbon monoxide is a colorless, odorless and tasteless poisonous gas that at high levels can cause loss of consciousness and death. —Alexandra Koch, FOXNews.com, 5 Feb. 2026

Kirk recognized that this crude conspiracism was poisonous to his project of popularizing the conservative cause. —Yair Rosenberg, The Atlantic, 4 Feb. 2026

No doubt one of them will bring home a plant that’s poisonous for felines. —Molly Aitken, New Yorker, 1 Feb. 2026

Chato, the male in the film, is talking about this mother wound as a poisonous notion of being an object instead of a subject, which is part of the patriarchy. —Georg Szalai, HollywoodReporter, 31 Jan. 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