If you don’t have enough water, your blood pressure is compromised and your intracranial pressure – the pressure of the fluids inside your skull, especially around your brain – might be affected. Also, blood vessels might react and the amount of blood going to your brain might change. These changes can give you a headache.
The problem with this kind of headache is that it’s quite specific. In other words, you could attribute the pain to a million reasons other than dehydration. So if you feel a headache, especially if it doesn’t go no matter what you do, there is a chance you actually need to drink some water. Just in case, any time you feel pain in your head, drink a glass of water.
It can do no damage and maybe it’s exactly what you need. If you want to take a painkiller, do it with a whole glass of water and you might be killing two birds with one rock.