Wouldn’t it be cool to have Twilight vampire eyes for Halloween?

Or deep violet eyes to match your purple sweater?

How about your favorite sports team’s logo on your eyes just for fun?

You can have all of these looks with decorative contact lenses (also called fashion contact lenses or color contact lenses, among other names). These lenses don’t correct vision—they just change the appearance of the eye.

But before buying decorative lenses, here’s what you should know:

* They are not cosmetics or over-the-counter merchandise. They are medical devices regulated by the Food and Drug Administration (FDA). Places that advertise them as cosmetics or sell them without a prescription are breaking the law.

* They are not “one size fits all.” An eye doctor (ophthalmologist or optometrist) must measure each eye to properly fit the lenses and evaluate how your eye responds to contact lens wear. A poor fit can cause serious eye damage, including: scratches on the cornea (the top layer of your eyeball); corneal infection (an ulcer on the cornea); conjunctivitis (pink eye); decreased vision; blindness.

* Places that sell decorative lenses without a prescription may give you few or no instructions on how to clean and care for your lenses.

Failure to use the proper solution to keep contact lenses clean and moist can lead to infections, says Bernard Lepri, O.D., M.S., M.Ed., an optometrist at FDA. “Bacterial infections can be extremely rapid, result in corneal ulcers, and cause blindness—sometimes within as little as 24 hours if not diagnosed and treated promptly.”

“The problem isn’t with the decorative contacts themselves,” adds Lepri. “It’s the way people use them improperly—without a valid prescription, without the involvement of a qualified eye care professional, or without appropriate follow-up care.”

FDA is aware that many places illegally sell decorative contact lenses to consumers without valid prescriptions for as little as $20.

You should never buy lenses from:

* street vendors

* salons or beauty supply stores

* boutiques

* flea markets

* novelty stores

* Halloween stores

* record or video stores

* convenience stores

* beach shops

* Internet (unless the site requires a prescription)

These are not authorized distributors of contact lenses, which are prescription devices by federal law.

* Get an eye exam from a licensed eye doctor (ophthalmologist or optometrist), even if you feel your vision is perfect.

* Get a valid prescription that includes the brand name, lens measurements, and an expiration date. But don’t expect your eye doctor to prescribe anime, or circle, lenses. These bigger-than-normal lenses that give the wearer a wide-eyed, doll-like look have not been approved by FDA.

* Whether you go in person or shop online, buy the lenses from a seller that requires you to provide a prescription.

* Follow directions for cleaning, disinfecting, and wearing the lenses, and visit your eye doctor for follow-up eye exams.

* See your eye doctor right away if you have signs of possible eye infection: redness; eye pain that doesn’t go away after a short time; decrease in vision.

Laura Butler paid $30 for her decorative lenses and $2,000 in medical bills. And she nearly lost an eye.

While at the beach in July 2010, Butler of Parkersburg, W.Va., bought a pair of blue contact lenses at a souvenir shop. The brown-eyed Butler was on vacation and just wanted to try a different eye color for fun, she says.

No instructions came with the lenses and the store didn’t sell contact lens solution. “They felt fine, but they moved around on my eyes and I had to adjust them with my finger,” says Butler.

As she was driving home the next day, Butler felt a sharp pain in her left eye. “It was such excruciating pain, I had to quickly pull over on the side of the road.” It took her 20 minutes to remove the contacts, she says, which had stuck to her eyes like suction cups. She drove home “with pain that was indescribable.”

A trip to the ER and then to an ophthalmologist gave Butler a diagnosis: corneal abrasion. “The doctor said it was as if someone took sandpaper and sanded my cornea,” she says. “He said he wasn’t going to sugar-coat it, that I could lose my eyesight or could lose my eye.”

Butler saw the doctor every day for 10 days and was under his care for seven weeks. “He took really good care of me and I didn’t get an infection,” says Butler. “But the pain was agonizing. I used to lay on the floor and roll back and forth in a fetal position for hours.”

Butler couldn’t see well enough to drive for eight weeks, had a drooping eyelid for five months, and still has decreased vision in her eye, she says. And she found out her optometrist could have ordered two sets of lenses for $50 and charged $60 for an eye exam.

Her advice: Don’t buy fashion lenses. If you do, “Take the time to go to the doctor, pay the extra money, and save yourself the agony.”