Does sunscreen expire?
Sunscreens are designed to remain at original strength for up to three years. This means that you can use leftover sunscreen from one year to the next.
Some sunscreens include an expiration date — a date indicating when they're no longer effective. Discard sunscreen that is past its expiration date. If you buy sunscreen that doesn't have an expiration date, write the date of purchase on the bottle and be sure to throw it out within three years. Also, discard sunscreen that has any obvious changes in color or consistency.
Keep in mind that if you use sunscreen generously and frequently, a bottle of sunscreen shouldn't last long. Generally, a liberal application is 1 ounce (30 milliliters) — the amount in a shot glass — to cover exposed parts of the body. 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 during one application.
To maximize protection, use a broad-spectrum sunscreen with an SPF of at least 30. Apply sunscreen generously, and reapply every two hours — or more often if you're swimming or perspiring.