Every sunscreen product is made up slightly differently, so each brand and type of sunscreen will have a different shelf life. In addition, excessive heat or sun exposure can cause the chemicals in the sunscreen to break down.
The safest thing to do would be to replace your bottle every two years. Most sunscreens, however, have a date of expiration stamped onto the label. There is usually a string of letters and numbers. If the string ends in a number, that is the year the sunscreen will expire (ex: 2004 = 4) Please see the attached link for further details http://www.uspharmacist.com/index.asp?show=article&page=8_1323.htm
The most common reason that sunscreens 'fail' is due to inadequate application. Please remember, apply the sunscreen 30 minutes before going outside, and reapply every 3-4 hours or earlier if sweating or swimming.
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