• Adults aged 18 to 75 years with a very severe to extremely severe medically refractory Chronic Daily Headache (CDH).

‣ Chronic: as defined by the International Headache Society Classification ICHD 3 (\>15 headache days a month for a period of at least 3 months)

⁃ Medically refractory: defined as failure of ≥3 headache preventative treatments.

⁃ Severely disabling (Migraine Disability Assessment \[MIDAS\] Tool score of ≥ 21 \[Grade IV\])

⁃ Score of 6 (very severe) or 7 (extremely severe) on the Global Assessment of Migraine Severity (GAMS) rating scale.

⁃ Head pain is at least moderate-to-severe intensity (\>5 on a Visual Analog Scale) in ≥50% of headaches.

• Diagnosis of CDH for ≥ 12 months before screening based on medical records and/or patient self-report.

• Headache frequency: minimum 15 monthly headache days (MHD) on average across the 3 months prior to screening

• Patient reports to their provider intolerance or insufficient response with their current preventive treatment (i.e. insufficient reduction in headache frequency, duration, and severity on a standard treatment on a generally accept therapeutic dose for 6 weeks).

• On a stable concomitant medication and headache preventive for the 3 months prior to screening

∙ During Baseline Period:

• Must have ≥ 15 MHD based on the patient self-maintained diary data during the baseline period.

• Must have demonstrated \>75% compliance with diary data for the 28 day duration period, or longer if their baseline period carries on past this point.

• Must continue to meet eligibility criteria when reassessed at baseline completion visit