I had once heard you should buy sunscreen new every year. This sounded like a marketing ploy to ensure you bought more. There is no ‘best-before’ date on the bottles.
I know from my student years of working in supermarkets, that the unsold summer-stock is merely boxed up and stored to be resold next year. Then the next, and the next. So supermarkets don’t apply this rumour to their stock.
So the next rumour I heard is the potency of sun-cream only fades if left in direct sunlight – so you should bin any left-over from your recent holiday.
Sounds like more marketing BS, albeit a more subtle amendment to the initial rumour.
I actually did a pretty thouough Yahoo search on this topic a few years ago (this was before Google). My wife insisted that sunscreens lost their effectiveness from season to season, so we had drawers full of perfectly good bottles. In my search I found lots of sites with scary warnings about skin cancer, and why SPF of 45 is really no better than SPF of 15, wnd why you should reapply from time to time, and why some sunscreens may be dangerous to children and furniture. But among all the warnings I couldn’t find a single site that claimed that sunscreen has any kind of shelf life. Not one; zero; zip. Not even the sites of the sunscreen makers.
Sunscreen is, in the U.S., an over-the-counter drug, and is regulated as such by the FDA. The rule for sunscreens and other OTCs is that stability testing must be done and an expiration date assigned; if the stability testing gives an expiration date of more than 5 years, no expiration date is required on the label.
If your bottle has no expiration date, and you’re in the U.S., you have years of service ahead of you. When I was a QC chemist for a sunscreen manufacturer, I NEVER saw significant degradation of any of the sunscreen ingredients. High-SPF sunscreens do tend to break down physically, however, but losing viscosity doesn’t make it less effective.
Remember: lay it on thick. The SPF is determined using a thick layer of sunscreen that most people don’t have the patience to apply. If it doesn’t leave your skin sticky and disgusting for a good 20 minutes, you need to put more on.
I’ve been using the same bottle of sunscreen for four years now. (It’s a big bottle.) Several times this summer I spent all day on the beach in full sun, and I didn’t even turn pink.
So my anecdotal evidence agrees with Nametag’s official evidence.
The Consumer Reports folks at my station did a test of sunblocks recently. You may be able to get the research at www.consumerreports.org.
They found that even when sunblock was heated for an entire summer (like you’d find if you left it in your car in the window) there was no significant degradation of protection from any given sunblock. So, don’t bother buying new stuff each season.