We study electric potential of a charge placed in a strong magnetic field B>>4.4x10^{13}G, as modified by the vacuum polarization. In such field the electron Larmour radius is much less than its Compton length. At the Larmour distances a scaling law occurs, with the potential determined by a magnetic-field-independent function. The scaling regime implies short-range interaction, expressed by Yukawa law. The electromagnetic interaction regains its long-range character at distances larger than the Compton length, the potential decreasing across the magnetic field faster than along. Correction to the nonrelativistic ground-state energy of a hydrogenlike atom is found. In the infinite-magnetic-field limit the modified potential becomes the Dirac delta-function plus a regular background. With this potential the ground-state energy is finite - the best pronounced effect of the vacuum polarization.
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