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Those cat-eye contacts may make your costume, but if you buy them, you might be putting your vision at risk.

Here’s the problem. While it’s easy to find decorative contacts online, for as little as $20, and some costume stores even sell them, the Food and Drug Administration (FDA) and Department of Homeland Security are warning that those vendors are not authorized to distribute contact lenses without a prescription, and they could be fined or face further prosecution. Contact lenses are regulated by the FDA as medical devices and therefore cannot be sold for any purpose — whether for cosmetic reasons or for vision correction — without a prescription from an ophthalmologist, optometrist or a specially licensed optician under who works under an eye doctor’s supervision.

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Why is Homeland Security worried about colored contact lenses? Because many of these lenses are being imported illegally, and are slipping past federal regulations for safety, which means they can injure eyes and vision. That’s why the FDA’s Office of Criminal Investigations (OCI) is teaming up with U.S. Immigration and Customs Enforcement’s (ICE) Homeland Security Investigations (HSI) and U.S. Customs and Border Protection (CBP) on a mission called Operation Double Vision, an effort to seize these counterfeit and illegal contact lenses. (Officials said they weren’t cracking down on the lenses over concerns about people changing their identities in an attempt to disguise themselves — at least for now.)

In a statement released about decorative lenses, the Department of Homeland Security recognizes that The Fairness to Contact Lens Consumers Act (yes, there is such a law) gives consumers the right to fill their prescription at businesses of their choice, including online shops. But if they buy lenses purely for decorative reasons, without a prescription, they won’t know what they’re getting and they could be putting their sight at risk.

When getting fitted with contacts, eye doctors measure the curvature of your cornea and prescribe the correctly curved lenses that hug the eye without squeezing or scratching the cornea. “If it’s too flat, [the lens] will move too much and scratch the cornea, allowing bacteria inside that can cause blindness,” says Dr. Richard Norden, a laser eye surgery specialist in Ridgewood, New Jersey.

And if contacts are too tight, not enough oxygen flows through, leading to dry and more easily torn corneas, which can also invite bacterial infections. Contact lenses that are not FDA approved may also use dyes that could be harmful to the cornea.

“Many of the places unauthorized to sell contacts do not explain how to sterilize contact lenses. People don’t know what they are getting and they don’t know when to take them out. If your eye starts to turn red, you must take them out right away to prevent early infections,” says Norden.

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If you absolutely must have decorative lenses for your costume, you are better off buying them through your eye doctor, who will custom fit them to your eyeball. You may end up paying more, but the prescribed lenses are worth the cost; once your cornea is scratched, you could lose your sight. In a 2011 study published in the journal Acta Ophthalmologica, researchers from Strasbourg University Hospital in France looked at contact wears who developed infections. They found that among cosmetic contact lens wears who developed a corneal infection, 60% ended up legally blind. Among non-cosmetic lens wearers who got infections, only 13% of ended up with significantly worse vision. The FDA warns that while eye doctors can make colored lenses and even cat-eyed lenses, but that anime or circle lenses that provide a doll-faced look are not yet approved.

So when it comes to your Halloween costume this year, you may want to think about the potential risks as well as benefits of having the perfect outfit; your eyes may thank you.