Dehydration symptoms: What to look for
March 16, 2026
Dehydration happens when the body loses fluids without replacing them. Anything that causes your body to lose fluids can lead to dehydration: spending too long in the sun with too little to drink, a sweaty workout without a break to refuel, or even illnesses like diarrhea.
Dehydration is also a side effect of some cancer treatments, such as chemotherapy, surgery or radiation. Treatment can trigger physical side effects, such as nausea, vomiting, diarrhea, fever, sweating, excessive urination, appetite loss and trouble swallowing. It can also cause cognitive changes that may reduce your awareness of thirst.
Here, I’ll share dehydration symptoms, what to do when you notice them and tips for staying hydrated, even when you aren’t feeling your best.
Key takeaways:
- Dehydration reduces your body’s blood volume and disturbs its electrolyte balance.
- Symptoms of dehydration may include thirst, dark urine color, reduced urination, rapid heart rate, dizziness, fatigue, dry mouth and headache.
- Dehydration may be treated by consuming water and electrolytes. Severe cases may require medical attention.
How can you tell if you are dehydrated?
Dehydration has signs and symptoms to look out for.
These include:
- Thirst
- Dark urine
- Reduced urine
- Rapid heart rate
- Dizziness
- Fatigue
- Dry mouth
- Headache
How does dehydration affect the body?
Dehydration reduces blood volume in your body. This can result in:
- Low blood pressure
- Feeling dizzy upon standing
- Muscle cramps
- Low urine volume
- Lower oxygen delivery leading to fatigue and shock
Dehydration can also disrupt your body’s electrolyte balance. Electrolytes are minerals that support the body's metabolic processes. Disrupted electrolyte balance can lead to:
- Muscle weakness
- Rapid breathing
- Confusion
- Lethargy
- Increased risk of seizure
- Excessive thirst
- Increased urination
What should you do if you are dehydrated?
If your dehydration is caused by not drinking enough water, correct it by drinking water when you feel thirsty.
Be sure to replenish fluids you're losing through vomiting, diarrhea or sweating.
If you are experiencing mild dehydration symptoms, drink small amounts of liquids that contain salt every 15 to 30 minutes.
When should you seek medical help for dehydration?
If you have any of the following symptoms of severe dehydration, seek medical attention.
- Weakness that prevents you from standing, along with skin that is cool, clammy, or looks blue or gray
- Rapid heartbeat, chest pain or trouble breathing
- Confusion
- Unable to consume fluids without vomiting
- Passing out or fainting
- Go more than 8 hours without urinating
If you have symptoms of severe dehydration or are unable to drink water, you may need to get an IV of water solution from an urgent care, emergency center or hospital.
Tips for staying hydrated
1. Know how much fluid to consume
According to the National Academies of Sciences, Engineering, and Medicine, females ages 19 to 70 should consume 2.7 liters, or about 11.5 cups, of total water from foods and beverages each day. Males ages 19 to 70 should consume 3.7 liters, or about 15.5 cups, of total water each day.
However, factors including age, body size, climate, activity level and even medical history can affect your recommended daily water intake. For example, adults ages 65 and older need to drink more water, as do women who are pregnant or breastfeeding.
Cancer treatment can also impact how much fluid your body needs. For example, people under age 65 who are receiving radiation therapy need about 30 milliliters of fluid per pound of weight, while those 65 and older need 25 to 30 milliliters of fluid per pound of weight.
Not sure if you’re drinking enough fluids? Track your fluid intake by tallying how many glasses of water you are drinking, using a labeled water bottle or even entering data into an app on your mobile phone.
2. Prevent dehydration during cancer treatment
When you are undergoing cancer treatment, it is important to meet your daily fluid needs. Dehydration during cancer treatment can cause health problems and even hospitalization and treatment delays.
For example, dehydration during cancer treatment can lead to:
- Worsened nausea and constipation
- Kidney strain and injury, especially when taking certain chemotherapy medications
- Low blood pressure, dizziness and weakness, which can increase your risk of falls
If you lose fluids, it is important to rehydrate with an oral rehydration solution. An oral rehydration solution is made of water, sodium, potassium and glucose that can be used by anyone with dehydration.
If your side effects are making it hard to stay hydrated, speak with your care team. They can prescribe medication to help manage these side effects.
Other ways to stay hydrated during treatment?
- Drink small amounts of water or fluid frequently.
- Eat foods with high water content, such as fruit, vegetables, yogurt, gelatin, popsicles and soup.
- Suck on popsicles or ice chips.
3. Avoid alcohol, soda and caffeinated drinks
It may seem that any fluid can help you stay hydrated. However, not all beverages are equally hydrating. For example, alcohol, soda and caffeinated beverages can promote fluid loss instead of helping you stay hydrated.
4. Start hydrating at the first signs of fluid loss or lack of intake
If you notice that you haven’t been consuming much fluid or find yourself experiencing any of the dehydration symptoms listed above, don’t wait to hydrate.
Natalie Carmouche is an outpatient senior clinical dietitian at UT MD Anderson.
Request an appointment at UT MD Anderson online or call 1-877-632-6789.
Don’t wait to hydrate.
Natalie Carmouche
Senior Clinical Dietitian