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

Trump’s populist allies have learned that their most poisonous ideas have a potent audience. —Jonathan Chait, The Atlantic, 25 Mar. 2026

Cycads can be lethally poisonous to dogs due to a toxin called cycasin, which causes abdominal pain, seizures, coma and liver failure. —Lauren Jarvis-Gibson, Miami Herald, 24 Mar. 2026

Morels are edible mushrooms with a honeycomb appearance and are hollow inside, unlike poisonous false morels. —Marina Johnson, Louisville Courier Journal, 24 Mar. 2026

The smoke kept rising and the clouds continued to throb with sickly yellow-and-black whorls until just after sunset, when the boiling mass of vapor suddenly turned pink and scarlet, like a poisonous flower opening its petals. —New York Times, 16 Mar. 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