A small number of allergy patients has headache as a primary symptom. You should suspect that allergy is the cause of your headache if it occurs only upon exposure to a certain environment or only after eating a certain food. An example of this allergic headache would be a person who develops headache when exposed to highly scented cosmetics. Cigarette smoke can also be a significant trigger of an allergic headache. When the headache is due to an inhalant, i.e., something breathed in, there is usually nasal congestion and blockage, watery eyes, or other signs of allergy.
Food allergies are less obvious.
You will have to keep a food diary and note the time of onset of each headache. A pattern may become apparent, with each headache preceded by ingestion of the same offending food. Common food allergens include milk, eggs, wheat, nuts, pork, chocolate, or shellfish. Avoiding the offending substance is the ideal way to prevent food allergies, but often this is not practical with inhaled substances that may be around you daily or during certain seasons. The use of an antihistamine is helpful.