Dehydration During Cancer Treatment: Fluid Goals & Warning Signs

Your body needs fluid to work as it should. When you don't have enough, that is dehydration. During cancer treatment it can build quietly over a day or two, and it can become serious. The National Cancer Institute is direct about this: when you have severe diarrhea, your body does not absorb enough water and nutrients, and dehydration "can be life-threatening, so tell your doctor or nurse if you have diarrhea."

The American Cancer Society lists the usual causes in people with cancer: nausea and vomiting, diarrhea, fever with or without infection, bleeding, procedures and surgeries, and trouble swallowing. Some people start out at higher risk — older adults, people with diabetes or kidney disease, and people who are having changes in thinking or memory that make it harder to notice thirst or to reach a drink.

MedlinePlus adds the same picture from the general medicine side: dehydration happens when you lose more fluid than you take in, and excessive sweating, fever, and increased urination from illness or medication all contribute.

Mild to moderate dehydration in adults, per MedlinePlus, looks like: feeling very thirsty, dry mouth, urinating and sweating less than usual, dark-colored urine, dry skin, feeling tired, and dizziness.

The American Cancer Society describes the same thing with a few extra signs worth checking at home: dry mouth and lips that make it hard to talk, a swollen or cracked tongue, skin that "tents" when you pinch it, a higher body temperature, headache, weight loss, sunken eyes, dark urine or making less urine, and less appetite.

Contact your cancer care team, per the American Cancer Society, if:

• You can't drink, or can't keep liquids down.
• Vomiting, diarrhea, fever, or trouble swallowing lasts more than 24 hours.
• Your urine is very dark, or you're making very little.
• You have had no urine for 8 hours or more.

Call 911 for: dizziness, weakness, or fainting when you stand up; confusion or disorientation; seizures; or a fast heart rate. MedlinePlus lists the same emergency picture — confusion, fainting, lack of urination, rapid heartbeat, rapid breathing, shock — and says to get medical help right away.

For diarrhea specifically, the National Cancer Institute grades it by how far above your normal you are. Grades 1 and 2, meaning up to six bowel movements above your normal daily number, "can usually be managed at home." Grades 3 and 4, meaning seven or more bowel movements above your normal daily number, "can be life-threatening and may require treatment in a hospital." That is why NCI suggests keeping a record of your bowel movements and agreeing with your team on what "normal" is for you.

There is no universal number in these sources, and it would be misleading to invent one. NCI's instruction is plain: "Drink lots of water or other fluids. Ask your doctor or nurse how much fluid you should drink." Your target depends on your treatment, your kidneys, your heart, and how much you are losing.

What the sources do agree on is technique. Sip small amounts often rather than large amounts at once. Cold fluids are often easier to get down. Keep something to drink within reach so you don't have to get up. Track what you actually drink in a day. Ice chips and popsicles count. So do fluid-rich foods: fruits, vegetables, soups, yogurt, and smoothies. Skip alcohol and caffeine.

If nausea and vomiting are the problem, NCI suggests water, broth, fruit juices, ginger ale, tea, and sports drinks through the day — and if you are losing weight you don't want to lose, drink fluids with calories instead of plain water.

If diarrhea is the problem, the advice shifts. NCI recommends clear liquids such as water or broth at room temperature, low-fiber foods like white bread, pasta, and canned fruit, and foods with sodium and potassium such as peeled and boiled potatoes, soup, bananas, applesauce, and crackers. Eat small, frequent meals instead of three large ones. Avoid alcohol, milk and dairy products, spicy foods, caffeinated drinks, dried beans, high-fat foods, fruit juices, and sugar-free gum or candies. Note that fruit juice helps with nausea and worsens diarrhea — match the advice to the symptom you actually have.

Mild dehydration usually responds to drinking more. Your cancer care team might suggest an oral rehydration drink to replace fluids, sugar, and electrolytes. When you can't take in enough by mouth, fluids are given intravenously. MedlinePlus notes that diagnosis typically involves a physical exam and vital signs plus blood tests for electrolytes and kidney function, and urine tests.

• National Cancer Institute: Diarrhea and Cancer Treatment. https://www.cancer.gov/about-cancer/treatment/side-effects/diarrhea
• National Cancer Institute: Nausea and Vomiting in People with Cancer. https://www.cancer.gov/about-cancer/treatment/side-effects/nausea
• MedlinePlus (U.S. National Library of Medicine): Dehydration. https://medlineplus.gov/dehydration.html
• American Cancer Society: Fluids and Dehydration. https://www.cancer.org/cancer/side-effects/eating-problems/fluids-and-dehydration.html

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