What You Should Know About Cancer and Dehydration

Dehydration happens when your body loses more fluids than it takes in. For most people, it’s uncomfortable. But when you have cancer — especially if you’re undergoing treatment — dehydration can affect your health, your energy, and your ability to stay on track with therapy.

Cancer itself, certain symptoms, and many common side effects of treatment can all contribute to fluid loss or make it harder to drink enough. If you ignore it, it can worsen fatigue, affect organ function, and even impact how well your body tolerates treatment.

Here, Dr. Youram Nassir at Cancer Care Institute explains the link between cancer and dehydration and what you can do about it.

Why cancer and cancer treatment increase dehydration 

There are several reasons cancer makes you more vulnerable to dehydration:

Vomiting and diarrhea

Chemotherapy and other treatments can irritate your digestive tract, leading to repeated vomiting or loose stools, which rapidly deplete fluids.

Poor appetite and eating problems

Nausea, mouth sores, or taste changes can make eating and drinking uncomfortable or unappealing.

Fever or infection

These increase metabolic demands and fluid loss.

Medications

Some drugs can increase urination or interfere with your thirst cues.

Reduced thirst or fatigue

Pain, fatigue, or emotional strain can make it harder to follow a regular fluid intake schedule.

Because these factors often overlap, people with cancer may become dehydrated more easily than others.

How do I know if I’m dehydrated?

Symptoms of dehydration can range from mild to serious, and because some signs overlap with treatment side effects, it’s important to pay close attention to changes in your body. Common indicators include:

• Dry mouth, lips, or eyes
• Feeling unusually thirsty or light-headed
• Dark or strong-smelling urine
• Fatigue or dizziness
• Headache, confusion, or irritability
• Rapid heartbeat or low blood pressure

Severe dehydration can lead to weakness, fainting, rapid breathing, and even kidney problems if not addressed promptly. Since your body is already under stress from your illness and treatment, dehydration can quickly make you feel even worse.

Why dehydration matters during cancer treatment

Hydration isn’t just about comfort; it’s about how your body functions on a fundamental level. Water plays many critical roles that are especially important when you’re fighting cancer:

• Helps transport nutrients and oxygen to cells
• Supports digestion, including how your body uses medication
• Assists kidney and liver function, which clears waste and drugs
• Regulates body temperature and maintains electrolyte balance

When you’re dehydrated, everything works less efficiently. It can make fatigue worse, interfere with your ability to tolerate treatment, and increase the risk of complications like infections, kidney impairment, and blood clots.

What can I do to stay hydrated?

Good hydration starts with awareness and habit, but sometimes it requires a bit more support when you’re going through cancer care. Here are practical steps you can take:

Drink regularly, even in small amounts

Sipping water throughout the day is often more tolerable than trying to drink large volumes at once — especially if nausea or mouth discomfort makes drinking difficult.

Include fluids with electrolytes

When vomiting, diarrhea, or sweating drains your fluids, turn to drinks with balanced electrolytes (like sports drinks or oral rehydration solutions).

Eat water-rich foods

Soups, smoothies, gelatin, watermelon, cucumbers, and other hydrating foods help boost your fluid intake without needing to drink large volumes of water.

Monitor your intake and output

If you’re keeping a fluid log for your care team, you’ll both have a clearer picture of how much you’re actually getting.

Recognize when home hydration isn’t enough

There are times when drinking fluids isn’t enough to restore proper hydration, especially if you’re experiencing severe vomiting, diarrhea, or treatment side effects.

When oral fluids aren’t enough — guided hydration support

Some patients find that intravenous (IV) hydration offers a more effective and immediate way to replenish fluids and electrolytes. IV hydration delivers fluids directly into your bloodstream under medical supervision, which can be particularly helpful if your body is struggling to absorb fluids orally due to nausea, mucositis, or other treatment-related challenges.

When to call us

You should contact your care team if you notice:

• Persistent vomiting or diarrhea
• Little or no urination for more than eight hours
• Extreme dizziness or confusion
• Rapid heartbeat or shortness of breath

These may be signs of significant dehydration or electrolyte imbalance and should be checked promptly.

You don’t have to manage this alone

Cancer treatment comes with enough challenges. Dehydration shouldn’t be one of them.

If you’re concerned about dehydration during your treatment, the team at the Cancer Care Institute is here to help. Call our office in Los Angeles or request an appointment online to talk about cancer and dehydration.