Doxycycline has been reported to relieve treatment-resistant new daily persistent headache in four patients.  The dose was 100 mg twice a day and was given for 3 months.  One of the patients responded after two weeks, but the average response time was 2 months.  The lead author of the study, Dr. Todd Rozen speculated that the reason for improvement is the fact that doxycycline is not only an antimicrobial drug, but also acts as an anti-inflammatory agent.