Dehydration: Headaches can be a symptom of dehydration, so make sure you're drinking enough before, during and after your runs. A basic rule of thumb is to drink 4-6 ounces every 20 minutes during your run. But use your thirst as your guide –- if you're feeling thirsty, you should drink. If you're running more than 90 minutes, make sure you alternate between water and sports drinks, so you're replacing electrolytes that you're losing through sweat.
Sunlight: Bright sunlight can be a headache trigger for some people. To help prevent headaches (and protect your skin and eyes at the same time), wear a hat with a brim and running sunglasses when running during the day. Try to run in the shade whenever possible.
Exercise headaches: Do you get throbbing headaches on both sides of your head during or after your runs? They may be exercise headaches. Exercise headaches occur more frequently when running in hot weather or at high altitudes, and are more common among runners with a personal or family history of migraines.
The exact cause of exercise headaches is unknown, but they are usually harmless and can be treated with medication. You may be able to prevent them if you avoid running during the hottest part of the day (which is a good idea to prevent heat-related illnesses, too) and make sure you're properly hydrating. If over-the-counter pain medications don't relieve your symptoms, talk to a health care professional.
While most exercise headaches are benign, rarely exercise headaches may be a sign of a serious underlying medical problem. If you start getting exercise headaches along with any of the following symptoms, you should talk to your doctor about it: Headache lasting longer than a day, loss of consciousness, rigid neck, double vision, vomiting, headache on only one side of the head, or the first episode of exertional headache occurring after age 40.
Get more advice about avoiding headaches after running.
"Exercise headaches." Mayo Clinic.com, accessed 6/9/10
"Migraine." National Headache Foundation, accessed 6/9/10