Want the piercing blue eyes of a Game of Thrones "White Walker" on Halloween, or perhaps the milky "blind" eyes of Arya Stark? Beware of colored contact lenses that are sold without a prescription because they can cause serious — even permanent — damage to the eyes.

Health officials and physicians say the lenses can lead to a host of ills: infections, abrasions, and ulcers. Some varieties can even lead to corneal hypoxia — when the cornea is starved of oxygen, said Anna P. Murchison, director of the Wills Eye emergency department at Jefferson.

"We worry about this every year," Murchison said.

» READ MORE: Beware of 'Halloween hand' and other scary holiday-related injuries

And don’t get Murchison started on another risky way to change eye color: using a needle to “tattoo” the whites of the eye. Canadian model Catt Gallinger is among the latest to try that ill-advised stunt, and now she is warning others that she suffered pain and blurry vision as a result.

Squeamish yet?

Murchison and her colleagues want all those would-be ghouls and goblins to know there is a safe way to alter eye color: tinted contact lenses sold with a prescription. That means an eye specialist has measured the curvature of the wearer's eyes, ensuring that the contacts fit properly. In addition, such lenses are made of safe materials and are approved by the U.S. Food and Drug Administration.

Still, the nonprescription varieties are easy to find this time of year, both from online retailers and at costume stores — even though their sale is illegal in at least one respect.

It is against FDA regulations to sell contacts that have not undergone agency review. This applies both to contacts that correct poor vision and to those designed solely to change eye color.

And even if a given type of lens is FDA approved, a retailer is running afoul of Federal Trade Commission rules by selling them without a prescription, said Alysa S. Bernstein, an attorney with the agency's division of advertising practices.

Legality aside, here is why nonprescription contacts can cause vision problems:

Contact lenses that lack FDA approval, generally the cheapest brands, can be made of impermeable materials. That means oxygen cannot reach the cornea, and someone who wears such lenses is literally suffocating the surface of the eye, causing it to become swollen and cloudy, Murchison said. Some cheap varieties also contain harmful substances such as lead, a neurotoxin, and chlorine, which can cause irritation. Others may be stamped with a design that can irritate the inside of the eyelid, she said.

Contacts that have been reviewed by the FDA, on the other hand, still can cause vision problems if sold without a prescription, meaning that they have not been fitted to the wearer's eyes. They can be too tight or too loose, leading to rubbing, inflammation, and infection, Murchison said.

Some contact-lens merchants are capitalizing on the popularity of the HBO series Game of Thrones, selling vivid blue lenses that allow the wearer to look like the mysterious White Walkers.

Murchison said she had watched one episode. As a physician, her first reaction was to be alarmed by all the violence. Then she thought of the potential for eye injury from ill-fitting cosmetic contacts.

Her advice: Get your eyes measured with a proper exam, and buy a pair that are approved by the FDA.

"Your vision is worth the price of a prescription," she said.

Or pick a different costume, instead.