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Prepositions after adjectives

How to use “poisonous” with prepositions

Poisonous is most commonly followed by to when describing harmful effects. Other prepositions usually belong to the wider sentence rather than directly to poisonous.

Quick answer

The usual pattern with “poisonous”

Use poisonous to to describe something harmful to people, animals, or systems. Poisonous for is uncommon and usually avoided. In, at, and about are generally separate parts of the sentence.

This plant is poisonous to cats.
Some gases are poisonous to humans.
The chemical proved poisonous to marine life.

Choose the pattern by meaning

These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.

1
poisonous to + living thing

Showing harmful effects

Use poisonous to when something can injure or kill people, animals, or organisms.

The berries are poisonous to dogs.
Lead is poisonous to children.
2
poisonous in + amount

Describing harmful content within something

In may describe where poison exists, not a fixed complement after poisonous.

The water is high in poisonous chemicals.
The air was filled with poisonous fumes.
3
poisonous for + activity

Talking about consequences in context

For occasionally appears with a broader meaning of harmful, but it is less idiomatic than poisonous to.

These fumes are dangerous for workers in closed spaces.
The substance is unhealthy for long-term use.

See the difference in context

The same subject can take different constructions because each sentence answers a different question.

Construction
Example
Meaning
poisonous to
The mushroom is poisonous to humans.
Standard pattern for harmful effects.
poisonous for
The chemical is dangerous for aquatic life.
For is more natural with dangerous or harmful than with poisonous.
poisonous in
The soil is high in poisonous metals.
In belongs to the noun phrase, not directly to poisonous.
poisonous at
The gas becomes poisonous at high temperatures.
At introduces a condition or time, not a fixed complement.
poisonous about
There is nothing poisonous about the dye.
About introduces a topic or quality, not a standard adjective pattern.

Common mistakes

Some combinations appear in real text but do not mean what learners often assume.

Using “poisonous for” instead of “poisonous to”

To is the normal preposition after poisonous when describing who or what is harmed.

✕ This plant is poisonous for dogs. ✓ This plant is poisonous to dogs.

Treating “in” as a complement after poisonous

In usually describes content or location separately from the adjective.

✕ The chemical is poisonous in children. ✓ The chemical is poisonous to children.

Using “about” after poisonous without a topic meaning

About is only used when discussing something, not for harmful effects.

✕ The berries are poisonous about pets. ✓ The berries are poisonous to pets.

Quick practice

Choose the expression that matches the intended meaning.

1. This mushroom is poisonous ___ humans.
2. The gas becomes poisonous ___ high temperatures.
3. There is nothing poisonous ___ the cleaning spray.

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Frequently asked questions

What is the standard preposition after poisonous?

The standard choice is to, as in “poisonous to humans.”

Is “poisonous for” correct?

It is uncommon. English usually prefers poisonous to or harmful to.

Can “poisonous” be used without a preposition?

Yes. For example: “The snake is poisonous.”

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