Doxycycline is the AAD's first-line oral antibiotic for moderate to severe acne — and clinical studies show 50–70% reduction in inflammatory lesions at 12 weeks, with 78% of patients achieving cure or significant improvement. But the correct dose, the 3-month duration rule, and the photosensitivity warning that most prescribers forget to mention make all the difference. This complete 2026 guide covers exact dosage by severity, the week-by-week results timeline.