Popping in a pair of novelty contact lenses might seem like a good way to make your Halloween costume extra creepy. But do you know what’s even scarier? The risks associated with wearing decorative lenses. They include infections, scratches, and even blindness!

Contact lenses are medical devices, usually worn to improve vision. The flexible plastic discs sit on top of the eye’s cornea, or transparent outer membrane. They work by focusing light as it enters a person’s pupil—the dark center of the eye—so objects appear clearer.

Because every person has a slightly different eye shape and size, all contact lenses—corrective or cosmetic—should be fitted by a professional. Without a doctor’s prescription, “it’s actually illegal to sell them,” says Thomas Steinemann. He’s an ophthalmologist, or eye doctor, at MetroHealth Medical Center in Cleveland, Ohio.

If contacts fit too snugly, tears can’t wash over the cornea to sweep away dust and bacteria. Contacts that are too tight can also scratch the cornea. Both of these issues can lead to eye infections, which can require surgery or even cause blindness. Steinemann recalls a 14-year-old patient who bought costume contacts to wear to prom. She got a severe eye infection and needed a corneal transplant. This procedure replaced her damaged cornea with a healthy one taken from someone who was recently deceased. Thankfully, the operation restored her vision.

Contacts are safe as long as you follow some guidelines, says Steinemann. Wear only prescription lenses, clean them regularly, take them out before bedtime, and don’t share them with friends.