Unless you’d like to keep the pirate look going, it’s best to avoid costume contact lenses this Halloween.

Like every October, health authorities and medical organizations want to remind you that the decorative, over-the-counter lenses are not only illegal, they’re also terrible for your eyes. And they're not telling tall tales. The lenses can cause infections, sores, scratches, vision-impairing scars, and even blindness. It’s easy to find eye-related horror stories from people who turned to black-market lenses to change the color, shape, or look of their eyes (some lenses even add logos to your eyeballs).

Just on Tuesday, USA Today reported the case of a Cleveland woman who got decorative lenses stuck to her eyeballs. The lenses were supposed to turn her brown eyes blue but instead made them swollen and red. She had to have them removed in an emergency room.

Still, decorative lenses have proven incredibly popular. The color-contact-lens market has at times been valued at $300 million, accounting for 15% of the total market for contact lenses.

People tend to pick them up from street vendors, beauty salons, flea markets, and novelty shops. But selling them over the counter is actually illegal. Contact lenses are considered a medical device and are regulated as such by the Food and Drug Administration. Anyone buying them needs to have a prescription for the lenses.

The dodgy, black-market lenses often turn out be contaminated, irritating, or simply the wrong size for your eyeball, causing injuries. If they’re too small for your eye or don’t allow enough air through, the lenses can cut off oxygen to the cornea—the clear surface of the eye. This corneal hypoxia can allow infections to fester as well as cause damage the corneal tissue.

And that’s just one of many potential problems. Things get even more dicey when people fail to follow basic contact-care instructions—like cleaning them occasionally and not sleeping with them or sharing them (which can spread infections).

In a 2005 medical report, a team of eye doctors laid out 12 consecutive cases of people arriving in medical centers with severe eye problems after using decorative lenses. The cases range from mild to extreme.

Shiver me timbers

Patient 1 was a 16-year-old girl who showed up with eye pain and redness after wearing a colored lens. She admitted to sleeping with the lenses as well as sharing them with her younger brother. Doctors treated her with artificial tears and an antibiotic ointment, and her symptoms quickly improved.

Patient 12, on the other hand, was not so lucky. After buying cat-eye lenses at a flea market, the 26-year-old man developed a severe, painful infection called Acanthamoeba keratitis. The infection is caused by a free-living amoeba running rampant in the cornea, which can be blinding—as it was in this case. He ended up needing a corneal transplant, and three months afterwards his vision was still 20/200, which is considered legally blind.

Patient 11 also suffered permanent vision loss. The 39-year-old woman went to the doctor with pain, redness, and blurred vision in her left eye after buying colored lenses at a hair salon. Her ophthalmologist quickly spotted an ulcer in the center of her cornea, which was positive for the pathogenic bacteria Pseudomonas aeruginosa. She was admitted to the hospital for intensive antibiotic therapy. But two months later, her best corrected vision in the affected eye was only 20/50.

Things could have been much worse for Patient 11 if she had been in the same boat as Patient 2. While Patient 11 told doctors that she regularly cleaned her black-market lenses, Patient 2—a 16-year-old girl—kept hers in dirty cases with obvious debris and continued to use them after doctors told her to stop.

When doctors analyzed the lenses, they found multiple bacteria normally found in the intestines (and feces) including Klebsiella species, Enterobacter aerogenes, and Pseudomonas aeruginosa. Two lenses also contained acanthamoeba—the amoeba that caused Patient 12’s blindness. One lens was positive for a fungal Fusarium species. The lens solutions and cases were positive for bacteria Serratia marcescens, Serratia fonticola, and Klebsiella species.

Patient 2 was seen multiple times for persistent infection and irritation but stopped going to appointments after six weeks.

Such poor hygiene and care practices can cause eye infections and damage with any type of contact lens. But doing so with sketchy fashion lenses is particularly dangerous.

If you really want to change the look of your eye, the FDA emphasizes that it’s very important to buy FDA-approved decorative lenses through your eye doctor or another reputable vendor—with your prescription.