Dehydration can occur in a matter of hours. Left unchecked, it can lead to serious problems—from an increased risk for stroke, kidney stones or impaired cognitive function to kidney failure and even death. Dehydration is particularly common in seniors. One of the main reasons is their diminished thirst sensation. Plus, as people get older, the kidneys lose the ability to conserve as much water.

The problem is that many people don’t realize that thirst is not the first (or only) sign of dehydration. You may not feel thirsty even though you need water. You may know some, but not all, of the early warning signs of mild dehydration: Feeling tired or lethargic…having a slight headache…experiencing mild muscle cramps…feeling a little foggy, light-headed or woozy when getting up from a chair…urine color that’s darker than your normal.

As dehydration worsens, you may experience greater light-headedness…a heart rate that feels fast or pulsing at rest as your heart pumps harder to get fluid to your organs…thirst…dry skin…a decrease in skin turgor—meaning if you pinch your skin it stays up for a few seconds.

To reverse moderate dehydration, drink a cup of water or a sports drink with electrolytes every 20 minutes for an hour—you should start to feel better within a few minutes. Important: If you feel dizzy and your heart rate is faster than usual, seek medical attention. You may need IV fluids for a faster rehydration than you can do on your own. If you feel like you’re going to pass out, call 911.

To prevent dehydration, make sure you take in enough fluids. Divide your weight in half and drink that number of ounces every day. Tip: 20% of your hydration comes from fresh fruits and veggies. Those highest in water content are celery, cucumber, lettuce and watermelon. Note: Be especially vigilant if you have a fever (you lose water through sweating), have diarrhea or vomiting, are exercising in hot weather or are taking antihistamines.