The U.S. Food and Drug Administration requires that sunscreen remain at its original strength for at least three years. That means you can use leftover sunscreen from one year to the next.

Some sunscreens include an expiration date — the date when they're no longer effective. Throw out sunscreen that is past its expiration date. If you get sunscreen that doesn't have an expiration date, write the date you bought it on the bottle. Throw out the bottle when three years have passed since that date.

To keep sunscreen in good condition, don't leave it out in hot weather or in direct sunlight. Put sunscreen bottles in the shade, wrap them in a towel or place them in a cooler. Throw out sunscreen if its color or texture changes significantly.

Keep in mind that if you use sunscreen often and generously, a bottle of it won't last long. In general, you should use about one 1 ounce (30 milliliters) on the parts of your face and body that will be in the sun. That's about the amount that fits in a shot glass. You might need to apply more, depending on your body size. If you have a 4-ounce (118-milliliter) bottle, you'll use about one-fourth of it for one application.