  We show that the axion decay constant does not get any correction at any
order by external magnetic fields. On the other hand, in the context of the
Wilsonian effective action under external magnetic fields, the axial currents
get a finite correction. We then calculate the effect of strong magnetic fields
$B>10^{18}G$ on the axion-nucleon coupling and find that if $B\gtrsim
10^{20}~G$ in strongly magnetized neutron and white dwarf stars, the emission
rate of axions is enhanced by several orders of magnitude.
