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

The plant, which is non-native and is for ornamental purposes, is poisonous to horses. —Alexa Herrera, CBS News, 16 June 2026

Not only are the leaves poisonous to humans and pets, but the vine itself is invasive and can spread disease to trees. —Samantha Johnson, Martha Stewart, 7 June 2026

The consequences ranged from hobbling Reconstruction to hastening the end of the Ottoman Empire to poisonous deflation, and the blundering response by governments helped shape the modern world. —Air Mail, 6 June 2026

Foxglove Foxglove features beautiful trumpet-like blossoms that are poisonous to cats. —Amy Deyoung, USA Today, 3 June 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