I wrote my first post on this topic in May 2007 when I forwarded a request from a trainer who wantend to learn from others experience on this topic. This today is my most read posting. In response to its popularity I added a second post.
In general you can bring back the cause of headache after running to three categories of causes.
- Muscle tension: When running you virtually use all muscles of your body including those in the neck and shoulders. Some people pull up their shoulders or are tensed when they run for other reasons (f.e. stress at work). This tension will lead to pain and strain on the muscles. This pain can radiate into the head leading to headache. This strain can also lead to the build up lactic acid in the neck muscles (just as will happen with the build up in you leg muscles while running). This acid will dehydrate the brain(cells) causing tension on the meninges which casues the headache just like with an hangover. Other casues of pain (for example problems with the cervical vertebrae) can also lead to this kind headache. If you think it might be the strain of your neck muscles: try to relax while running, let your shoulders hang or do some relaxing exercises before of during the training.
- Dehydration: When you run you loose water through perspiration. Of course you can compensate this for the large part by drinking during your exercise. You also loose electrolytes which are harder to replace. Both causes a disbalance of fluids in your brain and just like above this will cause a headache. Secondary: the loss of water can also result in a heat stroke / hyperthermia when it is no longer able to produce enough sweat to cool the body with perspiration. Here you can find advice on staying hydrated during running. And here the symptons and again some prevention tips of which the most important is, of course, drink!
- Bloodvessels expanding en narrowing: When starting and exercise the boody widens the bloodvessels to enhance the transport of oxygen and nutriants to the working muscles. After the exercise the vessels narrow again. A good cooling down will slow down this process reducing the risk of a headache.
"......a headache occurring after running or other similar exercise is more likely to be caused by the increased speed at which the blood is circulating..." [http://www.brainandspine.org.uk/]
And if you already have a headache during the exercise and not just afterwards the sudden widening of vessels and increase of bloodflow might cause it so do a relaxed warming up so your body slowly can get used to the increased pace it has to addept to.
So I hope this helps as a start if you are looking for some answers on this topic. Otherwise just google around a bit more on exertion headache. It is always a good idea to consult an expert (doctor) preferable someone with understanding of running.