• DEET-based bug sprays, such as OFF!, won't technically expire but they may become less effective over time.
• The same goes for bug sprays that use picaridin, oil of lemon eucalyptus (OLE) or para-menthane-3,8-diol (PMD).
• But if the active ingredient in your bug spray is ethyl butyl acetylamino propionate (IR3535), it may expire within 18 months.
• This article was scientifically reviewed by Josh Bloom, who holds a Ph.D. in chemistry. He is now the director of chemical and pharmaceutical sciences at the American Council on Science and Health in New York.
• Visit Insider's homepage for more stories.

As the weather heats up, bugs start to come out. Our best defense against the itchy, painful bites is bug spray repellent. But some bug sprays will expire eventually. Here's what you need to know.

How long bug sprays last

There is no one-size-fits-all answer here, but the general consensus is that bug sprays have a shelf life of around three years. At that point, you should probably throw them out – not because they will harm you, but because they might be less effective in repelling bugs.

Whether a bug spray expires or not depends on the active ingredient, says Laurence Zwiebel PhD, Cornelius Vanderbilt Chair in Biological Sciences at Vanderbilt University. Most active ingredients used in bug sprays are relatively stable and last a while. However, even if the active ingredients themselves don't expire, the overall formulation may become less effective over time.

Check the back of your bug spray bottle to see what chemical it uses. Here's how long common active ingredients will last:

DEET

The majority of popular bug spray brands such as OFF! contain N,N-diethyltoluamide, more commonly referred to as DEET, as their active ingredient. DEET doesn't expire. This chemical is quite stable and has a very long shelf life, says Zwiebel.

Picaridin

Picaridin is another chemical that is very effective at repelling bugs. Like DEET Picardin is a very stable chemical. Picaridin based repellents usually do not have expiration dates on the bottle. Brands that use Picaridin include Sawyer and Ranger Ready.

IR3535

IR3535 is a name commonly for ethyl butyl acetylamino propionate. Bug repellants with IR3535 often do have expiration dates, since this chemical is less stable than DEET or Picardin. The expiration date might be as soon as 18 months from the manufacture date. Avon and Coleman are two brands that use IR3535 in their formula.

Oil of lemon eucalyptus (OLE) or PMD

These similar oils are found in sprays like Repel and Off! Botanicals. The synthesized version of OLE is called PMD, which stands for para-menthane-3,8-diol. Repellants with this active ingredient are stable for long periods of time and likely won't have an expiration date on the bottle. REPEL is the most popular brand using OLE. You'll also find it in Murphy's Naturals bug spray.

How to tell if your bug spray is bad

If you try out an old spray and you notice that it smells funky or if it isn't working to keep bugs away from you, it's definitely time to buy a new bottle.

If you aren't sure how long you've had a bug spray for or you have questions about the formulation, it's best to call the company directly and ask them about the specific product in question. They will have the most up-to-date and formula specific information for you.

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Read the original article on Insider