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

Traces of plant toxins The chemical residues of this poisonous plant were found on 60,000-year-old quartz arrowheads. —Mrigakshi Dixit, Interesting Engineering, 7 Jan. 2026

The state also has an above average number of polluted waterways and more coal ash ponds — small, toxic water bodies near coal plants where poisonous refuse is discarded — than any other state. —Karl Schneider, IndyStar, 6 Jan. 2026

An herbalist who had a way with poisonous plants. —Literary Hub, 5 Jan. 2026

Others detect bitter tastes, tuning in to the large number of compounds in nature that are poisonous. —Linda Bartoshuk, The Conversation, 5 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