Aspirin/administration & dosage; Aspirin/adverse effects; Dipyridamole/administration & dosage; Dipyridamole/adverse effects; Headache/chemically induced; Stroke/drug therapy; Adenosine/metabolism; Aged; Cerebral Arteries/drug effects; Cerebral Arteries/physiopathology; Cerebrovascular Circulation/drug effects; Cerebrovascular Circulation/physiology; Dose-Response Relationship, Drug; Drug Administration Schedule; Drug Synergism; Drug Therapy, Combination; Drug Tolerance/physiology; Female; Fibrinolytic Agents/administration & dosage; Fibrinolytic Agents/adverse effects; Headache/physiopathology; Headache/prevention & control; Humans; Male; Migraine Disorders/complications; Migraine Disorders/physiopathology; Phosphodiesterase Inhibitors/administration & dosage; Phosphodiesterase Inhibitors/adverse effects; Stroke/prevention & control; Treatment Outcome; Vasodilation/drug effects; Vasodilation/physiology