Health experts warned Wednesday that people should not attempt the latest viral challenge -- walking over a pyramid of precariously balanced milk crates.

TikTok and other social media sites have been flooded in recent days by videos of people in the United States and beyond trying -- and mostly failing -- the climb the crates.

Most videos end with what looks like a painful fall onto the collapsing crates, or the ground, as onlookers film on their phones in what some dubbed an event in the "Hood Olympics."

"You're at risk for... hitting your head and getting a head bleed," said Dr. Chad Cannon, an emergency room doctor at the University of Kansas Health System.

If "you land on the milk crate, you will break your back and be paralyzed."

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Baltimore City Health pointed out that hospitals are already under strain from the pandemic.

"With Covid-19 hospitalizations rising around the country, please check with your local hospital to see if they have a bed available for you, before attempting the #milkcratechallenge," the official account tweeted.

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While the hashtag was readily searchable on Twitter and Instagram on Wednesday, searches on TikTok returned no results.

"This phrase may be associated with behavior or content that violates our guidelines. Promoting a safe and positive experience is TikTok's top priority," the search result page said.

The videos -- and photographs of some appalling injuries -- were reminiscent of an earlier internet sensation that had doctors tearing their hair out.

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The 2018 Tide Pod Challenge saw young people biting into a liquid laundry detergent packets.

Some social media users pointed out the likelihood that not all of those appearing in these videos in the United States would get free medical treatment.

"People doing this like they have the best health insurance" tweeted @ogmike.

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Others contrasted the willingness of people to attempt something so patently dangerous with the attitude to getting a coronavirus jab that has proven safe and effective.

"You'll do the milk crate challenge but won't get the vaccine. Got it," quipped 'Star Trek' actor George Takei.

Just over half of Americans are fully vaccinated against Covid-19, despite the shots being free and widely available.

Opposition to the jabs is driven by politics, distrust of government and antipathy towards science, and is particularly acute in poorer, more conservative parts of the country.