Diltiazem 2% ointmen
50% of fissures will get better on their own, without any intervention.
For the other half, the majority can get treated with these ointments.
Unfortunately, there is not a natural remedy that has been investigated that provides cure.
If the side effects are bothering you with the nitro ointment, Diltiazem has a lower side effect profile and higher cure rate. I'd suggest that.