Headaches are extremely common illnesses with a combined lifetime prevalence of 90-99% in Europe. Despite this high prevalence, there are persons who have never, in their whole life, encountered a headache.

The aim of the study is to identify factors that protect against headache by studying headache after provocation in people who never have had a headache (headache resistant) versus non-resistant controls.

The investigators will use isosorbide mononitrate as a pharmacological trigger of headache. The provocation is performed by investigators who are blinded to the grouping of the participants.

The investigators hope to contribute with novelty to the current understanding of headache pathophysiology and development of more efficient treatment of headache.

Inclusion Criteria:

• Headache resistant and non-resistant male participants from the Danish Blood Donor Study.
• Weight: 45 kg to 95 kg

Exclusion Criteria:

• Daily consumption of any medication.
• Consumption of any medication less than 12 hours prior to the study day.
• Headache on the study day or 48 hours prior to the study day.
• Amnestic or clinical signs of hypertension (systolic blood pressure >150 mmHg and/or diastolic blood pressure >100 mmHg) or hypotension (systolic blood pressure <90 mmHg and/or diastolic blood pressure <50 mmHg).
• Any severe cardiovascular disease, including cerebrovascular illness.
• Amnestic or clinical signs of current mental illness.
• Amnestic or clinical signs of current substance or drug abuse.
• Amnestic or clinical signs of any illness the responsible doctor considers relevant for participation in the study.

The investigators are blinded to the case-control grouping of the participants.