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

For this reason, these poisonous mushrooms are more commonly confused with other types of edible mushrooms like chicken of the woods and chanterelles (more on these in a minute). — Katie Hill, Outdoor Life, 6 Aug. 2026

Perhaps ducks had been used as bio-processors, feeding on toads and metabolizing their poisonous compounds so that humans could safely consume them. — Kimon De Greef, Longreads, 4 Aug. 2026

Not all pufferfish are poisonous, but the vast majority contain a lethal neurotoxin called tetrodotoxin in their internal organs and skin. — Teresa Mull, FOXNews.com, 1 Aug. 2026

The lionfish's poisonous spikes can badly hurt anyone who’s stung. — ABC News, 30 July 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