You might see a lot of scary sights on Halloween—most of them fun and harmless. But one freaky sight you want to avoid is an eye infection caused by illegal colored contact lenses—also called “fashion” or “costume” lenses.

You need a prescription

“Contact lenses of any type are medical devices that sit directly on your eyes,” says John A. Moran Eye Center ophthalmologist Bill Barlow, M.D. “And that means you need a prescription, a proper fit, and information on how to care for them before wearing. The point is to avoid some common dangers, including inflammation or infections—some of which can be serious.”

Risks outweigh rewards

Because of the multiple health risks involved, it’s actually illegal to buy contact lenses without a prescription (even if you don’t need any vision correction), so the vendors who are selling cosmetic lenses as a type of accessory or jewelry are breaking the law. The Food and Drug Administration even has a website where you can report the illegal selling of these lenses.

“While the sellers are the lawbreakers, their customers may end up paying the price with damaged eyes, so it’s important to be vigilant,” says Barlow.

Here’s how to safely wear costume contact lenses:

• Get an eye exam from a licensed ophthalmologist or optometrist, even if you think you have perfect vision.
• Get a valid contact lens prescription that includes a brand name, lens measurements, and an expiration date.
• Whether you buy the lenses online or in person, buy them from a seller that requires you to provide a prescription.
• Learn about and practice appropriate contact lens care.
• If your eyes start burning or itching while wearing your lenses, take them out immediately. See your eye doctor if you notice any pain, blurry vision, or discharge.

Plan early, plan safely. No one wants one night in costume to haunt them forever.