If you don't hydrate properly before, during, and after your run, you may be at risk for dehydration. Here are the symptoms to look out for and how to prevent getting dehydrated:
Early signs of dehydration include increased thirst; nausea; dry mouth; headache; reduced urine output, with dark yellow urine. Symptoms of moderate dehydration include extreme thirst; dry appearance inside the mouth; decreased urination, or lightheadedness. Serious dehydration can lead to cramps, chills and disorientation.
Not drinking enough to replenish water excreted through sweating, breathing, and eliminating waste.
The easiest way to avoid heat disorders such as dehydration is to keep your body hydrated. This means drinking fluids before, during and after exercise. The current fluid recommendations for runners say that they should "obey your thirst" and drink when their mouth is dry and they feel the need to drink. Drink before workouts and make sure you have access to fluids if exercising longer than 30 minutes. During longer workouts, some of your fluid intake should include a sports drink (like Gatorade) to replace lost salt and other minerals (electrolytes).
The only effective treatment for dehydration is to replace lost fluids. You should keep drinking fluids until you urinate. If your urine is a light lemonade color, that means you're re-hydrated. If it's a dark yellow, keep drinking. Someone who is severely dehydrated may need intravenous hydration, which can give them water and essential nutrients faster than drinking.
"Exercise and Fluid Replacement", ACSM Position Stand, American College Of Sports Medicine, Medicine and Science In Sports & Exercise, 2007.
Tamara Hew-Butler, DPM, Joseph G. Verbalis, MD, and Timothy D. Noakes, MBChB, MD, DSc, "Updated Fluid Recommendation: Position Statement From the International Marathon Medical Directors Association (IMMDA)," Clinical Journal of Sports Medicine, 2006;16:283–292)