Lots of people take their Halloween costumes to the next level by using colored or theatrical contact lenses, but the ophthalmologists are warning that the practice could lead to blindness.

According to Dr. John Rabins of Optom-Eyes, Inc., a scratch on the cornea or an ulcer could lead to permanent vision loss. Theatrical lenses pose a risk for such harm. Other unpleasant side effects are decreased vision and corneal infections.

"Contact lens prescriptions by law require an evaluation of the fit by a professional. So it's technically illegal to sell contact lenses without a valid prescription," Rabins told KXRM-TV

In 2005, a federal law was passed making it illegal for anyone other than a licensed eye care professional to distribute them, but it's still easy to find them online and in mall kiosks, flea markets and beauty salons.