Lots of us have done it: We’ve stretched our 30-day contact lenses to 32. Or 35. (Or 40? Please say no!) Maybe it was accidental — you were on vacation when you got to the last day. Or perhaps you told yourself it was a way to save money. With a few extra days on each pair, you could go through 11 pairs a year instead of 12.   

Whatever your motivation, it was a risky decision, says Matt Houck, O.D. He’s an optometrist in Portage, Michigan, with National Vision, the parent company of America’s Best Contacts & Eyeglasses — and a contact lens wearer himself.   

“Overwearing monthly contact lenses isn’t good,” he says. “If you wear them for 31 days, are you going to be in big trouble? Probably not. But every little thing you do that’s outside of your eye doctor’s recommendation raises your risk of a problem.”   

What, exactly, could go wrong? 

Three main issues come with wearing contact lenses past their throw-away date. As lenses get older:   

Proteins start to build up.  

Your tears contain proteins that get transferred to the surface of your contact lenses. As the proteins accumulate, your contacts will become less comfortable and will start to cloud up, compromising your vision. 

What’s popular right now? Savings.

Two pairs and a free, quality eye exam for just $79.95

FIND EXAM TIMES

Oxygen won’t get through as easily.  

Yes, your eyes need to breathe. Like all cells in your body, your corneas (the clear domes covering the colored part of the eyes) require oxygen to function. But because corneas don’t contain blood vessels, they have to get their oxygen supply mostly from the air.  

Contact lenses are manufactured to allow for this, but that’s exactly why there’s a disposal schedule: The makers base it on how long it takes the lens material to start breaking down.   

Pits and scratches can develop, which gives bacteria and viruses a zone to grow in. 

This is the most serious concern with overwearing your contact lenses, Dr. Houck says. Serious eye infections aren’t that common — every year, 1 in 500 contact lens users wind up with one, according to the American Optometric Association. You don’t want to be that person. Two extreme possible outcomes of such an infection are loss of vision and even loss of the eye itself. (If you thought wearing your contacts for extra days would save money, think about how much more costly a serious eye illness could be.)     

Any other hazards I should know about? 

Wearing contact lenses itself carries some risks, but they are more likely among people who don’t follow the wear and care of their lenses as directed. In other words, if you’re a committed contact lens wearer, you already need to look out for these concerning conditions (so don’t make it worse by not following instructions):   

Allergic pink eye (giant papillary conjunctivitis). This is an allergic reaction to something in the eye, which causes the eyelid to become rough, red, swollen, and sometimes bumpy inside. It’s more common in people who wear contact lenses, especially soft ones, and in people who don’t replace them often enough.  

Scratched cornea (corneal abrasion). Anything that gets in your eye can scratch the surface, or cornea. This includes the edge of a contact lens. It may feel like something gritty is in your eye. And you might have redness, watery eyes, pain, sensitivity to light, and a headache. Call your eye doctor right away or seek emergency care to keep it from becoming infected or inflamed.    

Eye infection (bacterial keratitis). When bacteria infect the cornea, that can cause many of the same symptoms as a scratched cornea. Wearing contacts raises the risk of this condition, especially if you wear them overnight or don’t care for them properly. (The bacteria gets trapped between your eye and the lens.) This is one of the risks that can lead to vision loss if not treated soon enough.   

Okay, you’ve convinced me! Is there anything else I might be doing wrong? 

If any of these bad habits sound familiar, Dr. Houck recommends you break them.     

You put your lenses in your mouth. You may be tempted to use saliva when you don’t have lens solution handy, but resist. You’re basically bathing your contact lenses in all the microorganisms that live in your mouth — and then delivering those bugs directly to your eyes. Dr. Houck recommends keeping extra bottles of multipurpose solution handy (in the car, at work, in your gym bag, etc.).   

You reuse your lens solution. Don’t “top off” the solution in your lens case. The solution is meant to disinfect your lenses, but that small amount will eventually be overcome by bacteria — not what you want. Whenever you put in your lenses, dump out the solution in the case and let the case air-dry before you use it again.  

You’ve had the same case for ... well, forever. There’s a reason new lens cases are often packaged with bottles of multipurpose solution, Dr. Houck says. The U.S. Food and Drug Administration recommends switching out your case at least every three months.    

You sleep with your contacts. Even if your lenses are approved for overnight use, most eye doctors don’t recommend wearing them during sleep. Your eyes need a break to allow oxygen to enter, to keep the tears and oils flowing to lubricate the eyeballs, and to avoid irritation in general. In particular, don’t sleep with contacts that are not FDA-approved for overnight wear. Overnight-use lenses are specially designed to be safer. For example, they contain silicone, which helps more oxygen reach the eye, while most other lenses do not.   

You rinse your lenses in tap water. Water from your faucet (or even distilled water) can carry a variety of bacteria, including Acanthamoeba, which causes acanthamoeba keratitis. This is a rare but serious infection that’s difficult to treat. Use disinfectant solution every time. Dr. Houck cautions that even wearing your contact lenses in the shower risks them coming into contact with potentially bacteria-laden water. (If you’re a swimmer, you can invest in prescription goggles. Swim shops and sporting goods retailers also often carry goggles with different reading powers to help you through your workout.)   

You wear your lenses all day. Ideally, you should wear glasses instead of contacts for a few hours each day — or whenever your eyes are tired or need a break. Glasses don’t carry the risks of contact lenses, and they tend to provide clearer vision. In fact, Dr. Houck says optometrists view eyeglasses as the ideal mode of vision correction.  

“Contact lenses are a secondary treatment and are considered cosmetic,” he says. “There are certainly people who love wearing them most of the time. I’m one of those people. As long as you wear them correctly, you can substantially reduce the risk of having a problem.”    

Of course, if you have any symptoms — redness, irritation, pain, discharge, a gritty feeling in the eye, or any new or sudden changes in vision — call your eye doctor right away. But you’ll go a long way toward avoiding that situation if you follow proper lens care.   

“It’s like wearing your seatbelt in the car. You don’t expect to get in an accident, but you wear it just in case,” says Dr. Houck. The same principle applies to contacts: Wear them as indicated by your doctor just in case. “Every person I’ve seen who got a bad infection after overwearing their lenses says they regret taking the risk,” he says.