This only applies to people who have a condition where they can't scratch an itch, like an injury or skin condition (otherwise just scratch it), but one new — albeit small — study suggests an interesting way to relieve an itchy arm or leg when you can't scratch: Stand in front of a mirror and scratch the opposite limb. Photo by John Benson.
So if you have an itch on your left arm, look yourself in the mirror and scratch your right arm instead. It may feel silly, but doing so can actually relieve the itch, according to the study, published in its entirety at PLOS One (which you can read, linked below.) Researchers conducted a pair of experiments where they artificially injected histamine into a subject's arm to incite itching, then put them in front of a mirror and asked them to scratch the opposite limb instead of the one they had injected. The results:
In both experiments, scratching the non-itching limb attenuated perceived itch intensity significantly and selectively in the mirror condition, i.e., when the non-itching forearm was visually perceived as the itching limb.
In essence, even though they knew they weren't scratching the itchy limb, as long as the visual illusion persisted that they were scratching the itchy arm, doing so relieved the itching. That visual illusion is important though, and if participants knew, even through the illusion, that they weren't scratching the right arm, it didn't work. The researchers say in the study:
The current study tested the hypothesis that "mirror scratching", i.e., scratching a mirrored non-itching forearm, can attenuate a circumscribed, experimentally induced itch. In line with our hypothesis, we observed a significant attenuation of itch by remote scratching only in an experimental condition in which the participant visually perceived the forearm being scratched as the itching forearm. Observing the non-itching forearm being scratched without the visual illusion did not result in itch reduction.
It's important to note that the participant groups here were quite small (26 men aged 19-38 in the first experiment, which went down to 20 after they accounted for experimental variables), and this was an experimentally induced itch to boot. Suffice to say your mileage may vary if you decide to give this a try yourself. On the bright side though, you don't really have anything to lose by trying to trick yourself this way, beyond, well, continuing to itch, and if you can fool yourself, you just might find relief.