Hydrogen peroxide, like many compounds, can expire. If you've ever poured hydrogen peroxide solution onto a cut and didn't experience the expected fizz, it's likely your bottle of hydrogen peroxide has become a bottle of plain water.

Hydrogen Peroxide Shelf Life

A 3% hydrogen peroxide solution stored at room temperature under normal conditions can be expected to decay at a rate of 0.5% per year.﻿﻿ Once you break the seal, it should be used as soon as possible because when you expose a peroxide solution to air, it starts to break down into water more rapidly. Likewise, if you contaminate the bottle—by dipping a swab or finger into it, for example—you can expect the effectiveness of the remaining liquid to be compromised.

So, if you have a bottle of hydrogen peroxide that has been sitting in your medicine cabinet for a few years, and especially if you've opened the bottle, assume that the compound is partially or fully decayed and no longer effective as a disinfectant.

Tips to Extend the Peroxide's Life

Don't open a new container of hydrogen peroxide until you're ready to use it and don't transfer it to a clear container. Like air, light reacts with peroxide by accelerating the rate of its decomposition. You can help extend the shelf life of your hydrogen peroxide by storing it in a cool location and in a dark container.

Why Peroxide Bubbles

Hydrogen peroxide begins decomposing into water and oxygen even before it's been opened. The chemical equation for this reaction is:

2 H 2 O 2 → 2 H 2 O + O 2 (g)

The bubbles formed during the decomposition of peroxide come from oxygen gas. Ordinarily, the reaction proceeds too slowly to be perceived, but when you pour hydrogen peroxide onto a cut or other surface containing a catalyst, it happens much more quickly. Catalysts that speed the decomposition reaction include transition metals such as iron in blood and the enzyme catalase.

Catalase is an enzyme found in nearly all living organisms, including humans and bacteria, and it acts to protect cells from peroxide by quickly deactivating the compound. Peroxide, even when produced by body cells themselves as part of the oxygen cycle, must be neutralized before it can cause oxidative damage.

But as peroxide undergoes oxidation, it destroys cells. This can be seen as bubbling. When you pour hydrogen peroxide on a cut, both healthy tissue and microbes are killed as the peroxide is attacked and begins to break down. Damage to healthy tissue typically repairs.

How to Test If Peroxide Is Still Good

If you're not sure whether that bottle of peroxide is worth keeping, there's a safe and easy way to test it: splash a bit into a sink. If it fizzes, it's still good. If it doesn't, it's time to replace the bottle.