Hi! My 16 Year Old Daughter Has Been On Minocycline

Hi! My 16 year old daughter has been on minocycline for about a year taking 150 mg a day for acne. Her acne is completely gone with that. We are now trying to ween her only giving her one 90 mg pill a day. Her acne is returning and she has some big hard cystic pimples. She wants to go back to 150 mg but my concerns are the long term effects from prolonged use of this antibiotic. She doesn't seem to have any minor side effects from it. How long should she stay on this and is there something else less risky that she could change to that would be better on her body? I don't know what to do. The dermatologist just says let's try to ween her, but it's not working. Any suggestions?