The caution came after analysts predicted 500,000 of the gizmos would sell in the UK before Christmas.
"When you text or use the iPhone you use far smaller muscles in the hand, which can fatigue quicker," The Sun quoted Tim Hutchful, of the British Chiropractic Association, as saying.
He said the problem is particularly evident in winter as less blood reaches muscles so they are stiff.
He added that RSI could spread to the arm and elbow if the 269 pounds gadget, which has a mobile, iPod, camera, and browser, is held at a right angle for texting.
Hutchful recommends users write short texts and keep their hands warm.
Apple refused to comment on the matter.