Updated December 16, 2014.

While most of us know the adage about drinking six to eight glasses of water each day, the truth of the matter is that many of us don’t follow through on it. Dehydration can sneak up on us quite quickly, and by the time we’re thirsty we may already be dehydrated, which can lead to a host of ailments including headaches. Fortunately for us, dehydration is highly treatable and easily preventable.

What Is Dehydration?
Dehydration occurs when the body’s electrolytes and water content are out of balance.

Simply put, dehydration occurs when the body loses more water (through things like sweat or urine) than it takes in. Dehydration is also more likely to occur in warmer climates, at higher altitudes, with increased physical activity, and when someone is ill. Dehydration can have serious effects on our body, leading to unconsciousness and death in extreme cases. More commonly the symptoms of dehydration include thirst, fatigue, weakness, muscle pain, wrinkled skin, increased heart rate and of course – headache.

What Is the Connection between Dehydration and Headaches?
Dehydration is a common underlying cause of headaches, and it’s also a common trigger for migraines. Headaches caused by dehydration can occur in the front or back of the head, or may be one-sided. Dehydration headaches also can be felt throughout the entire head, similar to a tension headache. A common symptom of dehydration headaches are an increase in pain when moving the head - especially during walking.

It’s not entirely clear why dehydration causes a headache.

Some studies have shown that blood vessels in the head may actually narrow in an attempt to regulate body fluid levels. Because this would make it harder for oxygen and blood to get to the brain, it results in a headache.

How Do I Avoid a Dehydration Headache?
Ensuring that your body gets enough fluids is the best way to avoid a dehydration headache. Some useful tips include:

Take in as much liquid as you need to never feel thirsty

Drink enough water so that urine is always a light yellow or clear color

Remember that not all fluids are created equal. Alcohol and drinks containing caffeine, such as coffee, actually function as a diuretic, which means they are dehydrating.

Recognize that if you are overweight, live in a warm climate, are at a higher altitude or engaged in strenuous physical activity – you may require more water intake than the recommended eight glasses of water

Keep in mind that consuming more fruits and vegetables is a way to increase your water intake – but it is not a supplement for drinking pure water.

Seek out shade when it is hot out, plan outside activities for cooler parts of the day, and hydrate in advance of any event.

How Do I Treat a Dehydration Headache?
To treat a dehydration headache, the body needs to become hydrated again. For mild dehydration, try to drink 16 to 32 ounces of water slowly. You should feel better within an hour or two. If you have not been able to eat, or the person experiencing the dehydration is a child, replace the water with fluid containing electrolytes, such as a sports drink or a rehydration beverage.

Stay in a cool environment and rest so that the body is able to rehydrate without sweating. If the dehydration is severe or the headache doesn't subside, go to a hospital so that you can be placed under the care of a doctor. In these instances, you will likely receive I.V. fluids to rehydrate quickly. This is most often necessary when the person is ill and unable to keep down any liquid.

However, keep in mind that prevention is the best treatment for a dehydration headache, so do what it takes to incorporate regular water drinking in your everyday routine.

<sub>Sources:

Torelli P, Manzoni GC. Fasting headache. Curr Pain Headache Rep. 2010 Aug;14(4):284-91</sub>