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

Having been burned badly by his phantom surplus in 2022, Newsom is obviously aware that a repeat would be politically poisonous. —Dan Walters, Mercury News, 13 Jan. 2026

Having been burned badly by his phantom surplus in 2022, Newsom is obviously aware that a repeat would be politically poisonous. —Dan Walters, Oc Register, 12 Jan. 2026

Anyone who may have eaten a poisonous mushroom is urged to seek medical help immediately. —Tim Fang, CBS News, 8 Jan. 2026

Both of these compounds are found in a poisonous onion called gifbol (Boophone disticha) that is still used by the region’s Indigenous hunters. —Andrew Paul, Popular Science, 7 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