riddle - In excess I'm lethal

I'm found in nature, but you'd never know the difference if I wasn't.
I'm used in plants, but not as the critical part.
I'm safe to consume, but you might think otherwise.
In excess I'm lethal, but then so is the light stuff!

Looking for a two-word answer.

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_{This puzzle is part of Recycling old answers.}

Answer

Are you

heavy water, which has two deuterium atoms instead of two hydrogen atoms

Explanation

I'm found in nature, but you'd never know the difference if I wasn't.

A few molecules of it are present naturally in light water, but if they were to go away, humans wouldn't notice that

I'm used in plants, but not as the critical part.

Plants use water. Wikipedia says that if they are watered with heavy water, they can survive, but cannot produce seeds, which is arguably the "critical" part of being a plant. Alternative suggested by commenters: Power plants use heavy water to prevent the reaction from going critical.

I'm safe to consume, but you might think otherwise.

Since it has something to do with isotopes, the average person might associate it with radioactivity and be afraid of it. But if people drink small amounts of it, it is not dangerous, and any accidental negative effects are not due to radiation.

In excess I'm lethal, but then so is the light stuff!

High doses of several grams of heavy water can kill humans and other animals. Standard (light) water kills humans if several liters are consumed at once.