Wearing colored contact lenses without a prescription may seem harmless, but beware: Costume contacts can severely damage your eyes and even cause permanent blindness if they’re not fitted by an eye specialist. And many costume contact lenses are sold without a doctor’s prescription, which is illegal in the United States.

If you’re wearing costume contacts for cosplay, Halloween or other events, here’s how to do it without damaging your eyes.

“Consumers need to know that permanent eye damage can occur from using non-prescription lenses,” says Thomas Steinemann, MD, a practicing ophthalmologist at MetroHealth Medical Center in Cleveland, Ohio. “Personally, I have seen far too many serious cases in both children and adults from using decorative lenses.”

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