(CNN) Just about everyone who's played with a projectile toy has gotten a version of the warning "You'll shoot your eye out!"

But in a report published Monday in the medical journal BMJ Case Reports , doctors warn that popular toy Nerf guns really can put eyes at risk.

The case report details three unrelated cases within three months treated in the Moorfields Eye Hospital accident and emergency department in London. Two adult patients had pooling of blood and inflammation in the eye after being shot with a Nerf gun. One 11-year-old patient had pooling of blood, inflammation and damage to the outer retinal layers of the eye.

"Significant Nerf gun ocular injuries are not reported in the literature, as far as we know," the researchers write.

In the emergency room at the eye hospital, doctors are used to attending to traumatic injuries. It was the extent of the injury from a children's toy that surprised Dr. Mukhtar Bizrah, the lead author of the study.

"Nerf guns are used by children, and I was not expecting to see blood," he said. All the patients complained of blurred vision and red eyes and were treated to reduce pressure inside the eye, and their symptoms resolved after a few weeks.

Bleeding under the white part of the eye, the conjunctiva, is common and not that dangerous, Bizrah said, but bleeding in the space between the cornea, the eye's outer layer, and iris, the circular colored portion of the eye, is far more dangerous. "One of the patients had angle recession, which is a risk factor for developing glaucoma down the line," he said.

All the injuries documented in the case study involved Nerf guns shot from a close distance.

One of the patients had been shot by "bullets" from an unlabeled brand, which have firmer heads than branded ones, the researchers said, noting that unbranded bullets are cheaper and tend to be preferred by parents. However, the authors said they could not verify whether branded bullets would have had caused less trauma.

Julie Duffy, senior vice president for global communications for Nerf's maker, Hasbro Inc., wrote in an email that Nerf foam darts and foam rounds are not hazardous when used properly. The guns should never be aimed at a person's eyes or face, and the products should never be modified, she said.

She added that darts that claim to be Nerf-compatible may not meet safety standards and regulations.

"NERF products are designed based on years of consumer insights and research, and undergo rigorous reviews and testing to assure that they are safe and fun to play with, and meet or exceed global standards and regulations."

Duffy urges parents to read product packaging and note age recommendations: "Ultimately, a parent or caregiver knows his or her child best and is best equipped to make decisions on what forms of play and entertainment are most appropriate for his or her child," she said.

Dr. Paul Kivela, president-elect of the American College of Emergency Physicians, said that the study raises concerns.

"Parents typically afraid of pellet guns. So soft-bullet guns give them a false sense of security that kids are safe. That is clearly not the case," said Kivela, who was not involved in the study. "The pressure of the projectile is important."

His advice: If you are going to play with projectiles of any kind, it's best to wear protective eyewear, even though that could be especially tough for children younger than 5.

"It's best to not have this toy around small children, even if there are older kids in the house," Kivela said. "It is also important to tell children to not shoot at a person's face or from a relatively close distance."

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Pediatric ophthalmologists at Nationwide Children's Hospital in Columbus, Ohio, were not at all surprised by the report. "We see eye injuries from projectile toys all the time, including Nerf guns. We know that there is always a risk of projectile toys hitting the eye," Dr. Marylou McGregor said. She also recommends using eye protection all the time when playing with such toys.

Patients with these kinds of eye injuries have to be monitored for the first week to ensure that there is no rebleeding, said Dr. Cate Jordan of Nationwide Children's Hospital. If pain and blood clotting persists, the entire anterior chamber has to be washed out. Jordan recommends the patient lay low and participate only in quiet activities for the first few weeks. Even after vision returns to normal, they have to be monitored every three months for the first year and then every year for longer-term risks like cataracts or glaucoma.