Nerf toy guns and their foam projectiles, as marketed and manufactured by Hasbro, are virtually harmless when used as instructed. But, as reported by CNN, a recent paper in the UK medical journal BMJ Case Reports is providing a reality check when it comes to using the mock weapons and off-brand ammo improperly.

Three unrelated patients were treated at Moorfields Eye Hospital in London with ocular injuries that were sustained as a result of being "shot" with Nerf guns. Two adults had bleeding and inflammation in the eye; one 11-year-old had bleeding, inflammation, and damage to the outer retinal layer. All three suffered what the paper described as "significant ocular trauma." Attending doctors treated their swelling, and all symptoms resolved within a few weeks.

So what happened? In the case of one patient, a Nerf play session went awry as a result of using non-licensed ammo that isn't subject to Hasbro's quality control measures and may be made of harder materials as a result. On their Nerf landing page, Hasbro cautions users to "never modify any Nerf blasters or other Nerf products. Use only the darts, water, rounds, and discs designed for specific Nerf blasters."

Pediatric ophthalmologists interviewed by CNN recommend that protective eyewear be used whenever anyone is playing with Nerf weapons. It's also advisable never to aim for the face when shooting and to avoid attempting to modify the weapons to shoot faster or farther.

[h/t CNN]