Doctors may have resolved the perennial up-or-down-toilet-seat debate for families with small boys in the house.

Leave it up, experts say.

Falling toilet seats are injuring an alarming number of recently potty-trained toddlers, a British study reports. The medical term for the damage – penis crush.

The trend for heavy wooden, ceramic and ornamental toilet seats is aggravating the problem, the study reports.

Dr. Joe Philip of Leighton Hospital warns that parents should be extra vigilant during the holidays.

"As Christmas approaches, many families will be visiting relatives and friends and their recently toilet-trained toddlers will be keen to show how grown up they are by going to the toilet on their own."

Writing in this month's issue of the British Journal of Urology, Philip and his colleagues report on four boys under the age of 4 who were admitted to hospital with injuries serious enough to require an overnight stay. All had been trained and were using the toilet on their own. They had lifted the seats, which had then fallen back down, crushing their penises. Fortunately, the injuries did not cause lasting damage.

"We are concerned that the growing trend of heavy toilet seats poses a risk not only to their health, but to their confidence," Philip says.

In Toronto, Dr. Marvin Gans, a pediatrician, has seen the injury in his young male patients. Often, the boys aren't lifting the seat to its fully upright position.

He suggests a little supervision, especially in unfamiliar bathrooms.

"The parent can go in and be sure the seat is fully up and everything is fine and then get out and allow the child to be independent," Gans says.

The study's authors are calling for manufacturers to design more seats that fall slowly, and for heavier seats to be banished from households with young boys.

Philip cites a recent market research report suggesting a "worldwide increase in the number of wooden and ceramic seats sold."

He suggested parents who refuse to leave the seat up should consider training their toddlers to hold up the seat with one hand.

Or not, Gans says.

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"The trouble is, when you're little, holding the seat with one hand requires a bit of a stretch.

"He just might fall in."