Approved by the Cancer.Net Editorial Board, 05/2018

A person can become dehydrated when they do not take enough fluid or lose too much fluid. Every cell and organ depends on water. Without it, the human body cannot function properly. The water in your body performs many tasks:

  • Transports nutrients and oxygen

  • Controls heart rate and blood pressure

  • Regulates body temperature

  • Lubricates joints

  • Protects organs and tissue, including eyes, ears, and heart

  • Creates saliva

  • Removes waste and toxins

Although it is possible to go for a long time without food, people cannot live without water for more than a few days. People receiving cancer treatment may be at a higher risk for dehydration due to treatment side effects, such as diarrhea and vomiting.

Signs and symptoms of dehydration

The longer you go without taking in enough fluid, the more dehydrated you will become. Thirst is one way your body alerts you to drink more. But waiting to drink until you are thirsty is often not enough because you can be dehydrated and not feel thirsty. Other possible dehydration symptoms include:

Severe dehydration can be life-threatening and needs immediate medical treatment. It can cause the following symptoms:

  • Extreme thirst

  • Low blood pressure

  • Fever

  • Rapid heartbeat

  • Lack of urination for more than 8 hours

  • Sunken eyes

  • Inability to sweat

  • Inability to produce tears

  • Disorientation or confusion

Talk with your health care team about any new symptoms or change in symptoms that you experience.

Causes of dehydration

You lose water every day through natural body functions. These include breathing, sweating, and going to the bathroom. Most people easily replace that fluid through drinking and eating. But certain conditions affect the body’s ability to stay hydrated. These include:

Diarrhea, nausea, and vomiting. Cancer treatment, including certain types of chemotherapy, radiation therapy, and surgery, can cause these side effects.

Fever. A high fever can cause dehydration. People receiving cancer treatment may be at risk for developing infections that can cause fever.

Age. Infants, children, and older adults are at a greater risk for dehydration. Young children pass water and electrolytes out of the body frequently. Electrolytes are minerals that help regulate the body.

As a person gets older, the body slowly loses the ability to conserve water. Older adults also are less likely to sense that they are thirsty and may not eat or drink enough, especially if they live alone. Illnesses, disabilities, and some medications may also increase dehydration.

Chronic illness. Many diseases, such as diabetes, cystic fibrosis, and kidney disease increase dehydration risk and/or the need for fluids. For example, people with uncontrolled diabetes urinate frequently. Some medications can also cause a person to urinate or sweat more than normal.

Environment. Living, working, and exercising in a hot or humid environment increase the need for fluids. People living at high altitudes, from 8,000 feet (2,400 meters) to 12,000 feet (3,700 meters) above sea level, also need more fluids. This is because their bodies lose water as they work to take in more oxygen.

Exercise. Everyone loses water through sweat. And people who engage in physical activity generally produce a significant amount of sweat. Even if you do not see sweat, you are likely sweating. The more you exercise, the more fluid you need to replace.

Other factors. Women and people who are overweight or obese are at a greater risk for dehydration.

Diagnosing dehydration

Your doctor can diagnose dehydration using several methods, including:

  • Taking your vital signs, such as your temperature, blood pressure, and pulse.

  • Doing a urine tests to check the level of dehydration or to find out what may be causing dehydration.

  • Doing a blood test to check factors such as your electrolytes and kidney function.

Treating dehydration

Relieving side effects, also called palliative care or supportive care, is an important part of cancer care and treatment. Treatment for dehydration depends on how severe it is. For mild dehydration, you might try the following:

  • Suck on ice chips or popsicles if you have trouble drinking or eating.

  • Apply moisturizer to cracked lips and medication to mouth sores. This can make drinking and eating less painful.

  • If you are able to drink, take in small amounts of fluid frequently instead of a large amount at one time. Drinking too much at once may cause vomiting.

  • Keep a water bottle with you at all times, and sip from it throughout the day.

  • Drink a large glass of water before bed and when you wake up each morning.

  • If you have diarrhea, choose drinks that have sodium and potassium to help replace these lost minerals.

  • If you have fatigue, keep ice and drinks within reach so you do not have to get up more often than necessary.

If you are not vomiting or experiencing diarrhea and are moderately dehydrated, your doctor may recommend that you drink an oral rehydration solution. Severe dehydration requires fluids to be given through a vein, also called intravenous (IV) fluids.

Preventing dehydration

The following tips can help you keep your body's fluid balance in check:

Drink lots of fluids. The amount of fluid needed each day to stay hydrated depends on your health and lifestyle. Ask your doctor how much water you should drink. If you dislike plain water, try drinking flavored water or adding a slice of lemon. Other fluids can also help, including milk, low-sugar juice, and low or caffeine-free tea.

Eat foods with high water content. Drinking water is the best way to hydrate. But many foods contain water and can also help replenish lost fluids. Choose foods such as lettuce (95% water), watermelon (92% water), and broccoli (91% water). Soups, popsicles, and yogurt also have high water content.

Manage side effects. Cancer treatment can cause nausea, vomiting, or diarrhea. Talk with your health care team about ways to prevent or minimize these side effects.

Do not wait to drink. Make a conscious effort to drink regularly and more often when you begin feeling ill, before you exercise, or before you go outside in hot weather. And make sure you are well hydrated beforehand to help lower your risk for dehydration.

Avoid foods and drinks that may contribute to dehydration. Drinks with sugar and caffeine, such as fruit juice, soda, and coffee, are not as effective at hydrating your body as low-sugar or low/caffeine-free beverages.

Related Resources

Cancer.Net Podcast: The Importance of Hydration

Side Effects of Chemotherapy

When to Call the Doctor During Cancer Treatment

More Information

MedlinePlus: Dehydration