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.

This puzzle is part of Recycling old answers.

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.

• I improved the formatting for you.You want to hide usually hide the explaination as well. – North Mar 29 at 14:25
• I spent like fifteen minutes answering and then I realized that I didn't even have a two word answer :( – North Mar 29 at 14:26
• Thank you @North, I now was able to rewrite the explanations to be much more clear, I had left them vague when they were visible. – rumtscho Mar 29 at 14:31
• It's important to know that it's not safe to consume in high amounts and it becomes toxic long before the regular stuff does. Once a majority of this substance replaces the normal stuff in your body, you will die from radiation-like sickness (I say "like" because it's not directly radioactive, but does interfere with cell division). – forest Mar 30 at 2:19
• @forest Fair point. – jafe Apr 1 at 14:29

I'm guessing

Hdrogen cyanide, or just cyanide.

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

Cyanide is found in nature, but in very small portions such as the core of apples, and fruits of the peach family. Wild almonds are also known to contain traces of cyanide (and considered a peach, or specifically te seed of such). However, cyanide is only toxic in seeds of said fruits when consumed in excessively large quantity. Bitter almonds are known for being cultivated with cyanide, so it must be treated and inspected before shipped to the public.

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

Like I mentioned, cyanide is found in the core of fruits in the rose family: this includes apples, peaches, cherries, nectarine, etc.

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

Okay so this might be a stretch, since cyanide isn't safe to consume, but really, really small doasges such as those found in the cores.

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

Pretty explainatory, cyanide kills at small and large doses.

Are you

carbon dioxide (CO2)

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

Invisible gas, breathing air with no CO2 in it makes no difference.

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

CO2 is needed for photosythesis to work, still the plant can survive longer with no CO2 than with no water or no sunlight.

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

Drinks containing CO2 or sodas are safe to consume, even if some people think it's bad for stomach, growth,..

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

Too much CO2 kills you, ask astronauts in apollo 13, divers, and so on. The light stuff is a gas.

• This fits amazingly well, but it's not the intended answer. – jafe Mar 29 at 22:47
• "I'm found in nature, but you'd never know the difference if I wasn't." when all the plants die, I think you would – Rambatino Mar 30 at 5:17