second course of isotretinoin - should i continue use isotretinoin or use somthing like Doxycycline?

i have had my first course of isotretinoin 4 years ago and after i finshed it the acne came back but not as it was before after 1 years my dosage was 40 mg per day my weight was 75 now i started my second course of isotretinoin with 50 mg per day like my dermatologist said my weight is 80 im now at final mounth (6) my face is cleared also my chest but top of my back is kinda breaking out so should i continue use isotretinoin or use somthing like Doxycycline