Sometimes, you just don't realize your sunscreen is expired until you need it.

You know how important sunscreen is. It's why you were about to put it on. Wearing sunscreen every day — especially if you plan to be out in the sun — doesn't just reduce skin aging. It's an important step in preventing sunburns and reducing your risk of developing skin cancer over time.

But, like most everything with an expiration date...you question it now and then. Can sunscreen really expire?

Unfortunately, yes. Sunscreen does expire — even if it there's no date listed on the bottle.

And while you might be a little loosey-goosey with the freshness of your other skin care products here and there, you really shouldn't be with your sunscreen.

When does sunscreen expire?

Regardless of type or SPF, all sunscreens expire. Many document the expiration date somewhere on the bottle, but not all.

The good news, though, is that sunscreen is regulated by the FDA. And one of the standards the FDA holds manufacturers to is that sunscreen must remain stable and effective for at least three years.

So, if the expiration date isn't listed on the bottle, rest assured that it can be used for up to three years...as long as you can remember when that is, of course. Since you'll likely have a hard time with that, consider writing the purchase date on the bottle and throwing it out after three years.

The bottom line: Yes, sunscreen can expire. The FDA requires sunscreen to have a shelf life of at least three years. Always be sure to check the expiration date before using sunscreen. When in doubt, assume it's expired and start fresh.

Why does sunscreen expire?

All sunscreens contain active ingredients that help prevent UV rays from damaging your skin. They also contain inactive ingredients that contribute to the consistency of the product, such as emulsifiers, and/or stability of the active ingredients, such as preservatives.

Both the active and inactive ingredients found in sunscreen can degrade over time, affecting how well the sunscreen works.

For instance, an active ingredient can break down to the point where it no longer actually helps protect your skin from harmful UV rays. And, as an inactive ingredient degrades, the consistency of the product can change — affecting spreadability, as well as whether the active ingredient is able to remain active in solution.

A less effective sunscreen means reduced protection from the sun. This is why sunscreens and their expiration dates are regulated by the FDA.

What's more is that sunscreen can actually expire prematurely if stored improperly.

Sunscreen should be stored in a cool, dry place. If left in the heat, such as in a hot car, or in direct sunlight for extended periods of time, ingredients can degrade more quickly and become ineffective before the expiration date.

Even if your sunscreen hasn't passed its expiration, replace it if you notice changes in color, smell or consistency — as these are signs of premature expiration.

The bottom line: The ingredients found in sunscreen degrade over time, reducing the effectiveness of the sunscreen. They can also degrade prior to the expiration date if exposed to prolonged heat or sun.

Why you shouldn't rely on expired sunscreen

For a product as important as sunscreen, you want to be sure that what you're using is effective. Remember, sunscreen isn't just about preventing skin aging and painful sunburns. It's an important part of reducing your lifetime skin cancer risk.

This means taking a sunscreen's expiration date seriously.

Before your sunscreen hits this date, its ingredients are considered stable and effective — aka, able to help protect your skin from damaging UV rays.

When sunscreen is expired, there's simply no way to know if it is still protective or provides the benefits promised in its labeling.

And since unprotected exposure to the sun increases a person's risk of developing skin cancer, the FDA advises to replace expired sunscreen rather than try to use it.

The bottom line: There's no guarantee that expired sunscreen is still effective at protecting your skin from the sun's harmful UV rays. Always replace expired sunscreen.