More than 1.3m people have bought a Blackberry
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Trendy handheld Blackberry devices could cause damage to users' thumbs, doctors are warning.
Sales of the £200 gadget, which can be used to email, page and phone, have boomed in recent years with celebrities such as David Beckham using them.
But US and UK doctors said repetitive use could cause arthritis or harm tendons in the thumb.
Research In Motion, which manufactures the Blackberry, pointed out that as yet no-one had come forward with problems.
More than 1.3m people have bought a Blackberry since they came on to the market, helping it to corner the top-end of the wireless communications market.
Sean Hughes, professor of orthopaedic surgery at Imperial College London, told BBC News prolonged use could potentially cause wear and tear on the thumb.
"People who use them a lot could suffer from osteoarthritis.
"The thing is the thumb is designed flex and rotate in all directions, it works differently from the fingers.
"The joint at the bottom of the thumb allows it to move like this and tapping away could cause it to become sore."
He said he had not heard of any cases to date however.
His warnings come three years after the British Chiropractic Association said regular text messaging could cause repetitive strain injury.
Concerns
US experts have also raised concerns about repetitive Blackberry use. Professor Alan Hedge, director of the human factors and ergonomics research group at Cornell University in New York, said in the early 1990s people had complained of sore thumbs from playing computer games too much.
"It is the same principle. The thumb is not very flexible and repetitive use of it can lead to damage to the tendon on the outside of the thumb.
"The Blackberry is much more demanding than texting on phones because users tend to use them to type out emails. Maybe they should be limited to 50 words or so."
Professor Hedge warned it was quite easy to use the devices too much.
"Really you should not be making more than a few hundred thumb movements a day, so that does not amount to that many words when you think about it."
But Tilly Quanjer, a spokeswoman for Research In Motion, added: "The opinions about thumb-typing that were voiced should be taken in the context that they are theoretical and that the medical professionals referenced in those reports have admittedly not encountered or become aware of any actual cases themselves."
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The thing is the thumb is designed flex and rotate in all directions, it works differently from the fingers
