Hypersensitivity to Phosphodiesterase 5 Inhibition in Post-Traumatic Headache

Inclusion Criteria: * Age 18 to 65 years of age upon entry into screening * History of persistent headache attributed to mild traumatic injury to the head for ≥ 12 months and in accordance with the International Classification of Headache Disorders, 3rd Edition (ICHD-3) * ≥ 4 monthly headache days on average across the 3 months prior to screening * Provision of informed consent prior to initiation of any study-specific activities/procedures Exclusion Criteria: * \> 1 mild traumatic injury to the head * History of any primary or secondary headache disorder prior to mild traumatic injury to the head (except for infrequent episodic tension-type headache) * History of moderate or severe injury to the head * History of whiplash injury * History of craniotomy * History or evidence of any other clinically significant disorder, condition or disease (except for those outlined above) than, in the opinion of the site investigator, would pose a risk to subject safety or interfere with study evaluation, procedures or completion * The subject is at risk of self-harm or harm to others as evidenced by past suicidal behavior * Female subjects of childbearing potential with a positive pregnancy test during any study visit * Cardiovascular disease of any kind, including cerebrovascular diseases * Hypertension (systolic blood pressure of ≥150 mmHg and/or diastolic blood pressure of ≥100 mmHg) prior to the start of infusion on the experimental day * Hypotension (systolic blood pressure of ≤90 mmHg and/or diastolic blood pressure of ≤50 mmHg) * Initiation, discontinuation, or change of dosing of prophylactic medications within 2 months prior to study inclusion * Intake of acute medications (e.g. analgesics, triptans) within 48 hours of infusion start * Baseline headache intensity of \>3 on an 11-point numeric rating scale (0 being no headache, 10 being the worst imaginable headache) * Baseline headache with migraine-like features or self-reported baseline headache that mimics the subjects' usual headache with migraine-like features