Doxycycline is the most prescribed oral antibiotic for acne in the USA — but most patients stop too early, just before it starts working well. First visible improvement typically appears at weeks 3 to 4, meaningful clearance by weeks 6 to 8, and maximum response at weeks 12 to 16. This complete guide covers the week-by-week timeline, correct dosage, how long to take it, and the anti-inflammatory mechanism most patients never hear about.