This is actually on purpose to additionally stage realism - the medikit cannot be used on the soldier himself because of hesitation. In real life, people hesitate before they poke themselves with a sharp and painful needle.
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In the game, this hesitation is so long that it would take too many TU's to ever overcome this hesitation factor.
But even so, the Medikit is a little bit buggy, because it seems to have unlimited ammo. Can you also heal aliens with a medikit??? :?