In many cases, the exact causes of headaches are unknown, as are the mechanisms that cause them. Two factors thought to play a role include inflammation, spasm or stretching in the scalp, the membranes that cover the brain, and in the facial or jaw muscles; and a temporary distortion of blood vessels that supply the brain, in which these vessels tighten, relax and expand. When these blood vessels enlarge, they can press against nerves and cause pain.
In addition, certain foods and additives have been found to cause headaches. These include monosodium glutamine, which is commonly used at some Chinese restaurants, in frozen foods, lunchmeats, and canned and dry soups; nitrates, which are chemicals used to preserve meats such as bacon and sausage; and tyramine, which is found in hard cheeses, peas, navy and lima beans, alcoholic beverages and chocolate.
Headaches can also occur alongside sinusitis, a fever or due to withdrawal from caffeine.