Approved by the Cancer.Net Editorial Board, 01/2020

Dehydration happens when a person does not take in enough fluid or loses too much fluid. Your cells and organs depend 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 the eyes, ears, and heart

  • Creates saliva

  • Removes waste and toxins

If you are receiving cancer treatment, you may be at a higher risk for dehydration due to side effects, such as diarrhea and vomiting.

What are the 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 fluid. However, sometimes you can become dehydrated without feeling thirsty. Other possible dehydration symptoms include:

  • A dry or sticky mouth or a swollen tongue

  • Fatigue or weakness

  • Irritability

  • Dizziness or lightheadedness

  • Nausea and vomiting

  • Headaches

  • Constipation

  • Dry skin

  • Weight loss

  • Dark yellow urine or a decrease in urination

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.

What are the 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. They may not eat or drink enough, especially if they live alone.

  • Chronic illness. Many diseases -- such as diabetes, cystic fibrosis, and kidney disease -- increase dehydration risk and 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. Exercise can make you sweat more. Even if you do not see sweat, you are likely sweating. The more you exercise, the more fluid you need to replace.

How is dehydration diagnosed?

Your doctor can diagnose dehydration using several methods:

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

  • Testing your blood for factors such as your electrolytes and kidney function

  • Testing your urine for the level of dehydration or to find out what may be causing dehydration

How is dehydration treated?

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

  • 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.

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

  • Keep ice and drinks within reach so you do not have to get up as often, if you are tired.

You doctor may recommend an oral rehydration solution if you are not vomiting or experiencing diarrhea. In this case, you may be moderately dehydrated.

Your doctor may prescribe fluids to given directly through a vein, also called intravenous (IV) fluids. In this case, you may be severely dehydrated.

How can dehydration be prevented?

The following tips can help 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, treatment, 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 caffeine-free tea.

Remember to avoid foods and drinks that may contribute to dehydration. Avoid alcohol. Choose drinks with low sugar and low or no caffeine. Water is often a better choice than fruit juice, soda, or coffee.

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). Soup, 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 reduce these side effects and any concerns you have about dehydration.

Monitor your environment and activity. Do not wait to drink water or other fluids. Make a conscious effort to drink regularly. Drink more often before you exercise and before you go outside in hot weather. During an illness or if you are feeling unwell, be proactive and drink water to stay hydrated in order to help your recovery.

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

American Cancer Society: Dehydration and Lack of Fluids