Is doxycycline going to mess me up? New cystic acne diagnosis

I have had mild to moderate adult acne my hole life that has been mostly well controlled with topical meds although it does flare in the summer and dry out in the winter.

This past few months my skin has been really irritated with a continuous break out in my t zone and 1-2 lumps without a clear head and pustules. My dermatologist thinks this is now cystic acne and wants me on a 30 day script of doxycycline.

I believe him but my skin doesn’t look nearly as bad as the pictures and I’m worried now what an entire month of antibiotics is going to do to my stomach and gut.

Any advise? Is 30 days a lot for acne? What can I do to minimize the impact on my body?