Hydration Guide for Cold & Flu
"Drink plenty of fluids" is the most common medical advice for colds and flu - and there's solid science behind it.
Quick answer
Hydration Guide for Cold & Flu
Cold and flu drive fluid loss through fever, sweating, and nasal discharge, so staying well hydrated matters during illness. If vomiting or diarrhea is present, oral rehydration solution (ORS) beats plain water: WHO/UNICEF low-osmolarity ORS (245 mOsm/L; sodium 75, glucose 75 mmol/L) absorbs via SGLT1 cotransport and cut the need for unscheduled IV fluids by about 33%.
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Medical Disclaimer: This information is for educational purposes only. Influenza can cause serious complications. See your doctor if symptoms are severe, especially if you are in a high-risk group.
When you're fighting a cold or flu, your body loses fluids through fever-induced sweating, nasal discharge, and increased respiratory water loss. A runny nose alone can cause 200-300ml of daily fluid loss. The Mayo Clinic estimates that flu patients need 1-2 liters of additional fluid per day above their normal intake. Proper hydration thins mucus secretions, supports immune cell activity, and helps your body maintain the elevated temperature needed to fight infection.
Why Hydration Matters for Cold & Flu
Mucus Thinning
Adequate hydration keeps mucus thin and easier to expel. Dehydration causes thick, sticky mucus that clogs sinuses, worsens congestion, and creates an environment for secondary bacterial infections.
Immune Cell Function
Your lymphatic system, which transports immune cells, requires adequate fluid to function. White blood cells, antibodies, and cytokines all travel through water-based blood plasma.
Sore Throat Relief
Warm fluids soothe inflamed throat tissue and help maintain the moisture barrier in your airways. Hot beverages also create steam that loosens nasal congestion.
Faster Recovery
Hydrated patients recover faster. Water helps remove metabolic waste from the immune response, reduces fatigue, and supports the liver in processing medications like antivirals and decongestants.
Hydration Guidelines
Common cold: Add 750ml-1L to normal intake
Even without fever, nasal drainage and mouth-breathing increase fluid loss. Aim for at least 2.5-3.0 liters total. Warm fluids like tea and broth are especially soothing.
Source: Mayo Clinic
Flu with fever: Add 1-2L to normal intake
Flu typically involves higher fevers than colds, dramatically increasing fluid needs. Target 3.0-4.0 liters daily, using a mix of water, ORS, and warm beverages.
Hot liquids for congestion relief
A study in the journal Rhinology found that hot beverages improved nasal airflow and reduced congestion symptoms more effectively than room-temperature drinks.
Source: Rhinology Journal
Avoid dehydrating beverages
Limit coffee, energy drinks, and alcohol during illness. While moderate caffeine is acceptable, these beverages can worsen dehydration. Stick primarily to water, herbal tea, and broth.
Warning Signs to Watch For
Signs of Dehydration
- Thick, dark-colored nasal mucus
- Dry, cracked lips and dry cough
- Dark yellow urine or infrequent urination
- Extreme fatigue beyond typical cold/flu tiredness
- Headache that worsens despite pain medication
- Dizziness when getting out of bed
Practical Tips
- Drink warm chicken broth or soup - it provides fluids, sodium, and anti-inflammatory compounds
- Sip hot water with honey and lemon for throat relief and hydration
- Use a humidifier to reduce respiratory water loss while sleeping
- Eat popsicles or frozen fruit bars if nausea makes drinking difficult
- Alternate between hot and cold fluids based on what feels soothing
- Set a timer to drink 200ml every hour - appetite and thirst decrease when sick
- Have someone prepare fluids for you when you're too tired to get up
When to Contact Your Healthcare Provider
- Difficulty breathing or shortness of breath
- Persistent chest pain or pressure
- Inability to keep fluids down for more than 8 hours
- Fever above 39.4°C (103°F) lasting more than 3 days
- Symptoms that improve then suddenly worsen (possible secondary infection)
- Signs of severe dehydration (confusion, no urination, rapid heartbeat)
Helpful Tools
Use our calculator to find your personalized hydration needs during illness.
Water Intake CalculatorSources & Citations
- 245 mOsm/L (old: 311) — WHO/UNICEF reduced-osmolarity ORS has a total osmolarity of 245 mOsm/L, down from the older 311 mOsm/L formula[1]
- Na 75, glucose 75, K 20, Cl 65, citrate 10 mmol/L — WHO low-osmolarity ORS contains sodium 75 mmol/L, glucose 75 mmol/L, potassium 20 mmol/L, chloride 65 mmol/L, and citrate 10 mmol/L[1]
- OR 0.61 (95% CI 0.47-0.81) — Reduced-osmolarity ORS cut the need for unscheduled IV fluids versus standard WHO ORS[2]
- ~33% reduction — WHO/UNICEF report the new ORS reduced the need for unscheduled supplemental IV therapy in children[1]
- OR 0.71 (95% CI 0.55-0.92); WHO ~30% — Reduced-osmolarity ORS reduced vomiting versus standard WHO ORS[2]
- 2 Na+ : 1 glucose — ORS works via SGLT1 cotransport, which carries sodium and glucose together so water follows osmotically; plain water lacks this drive[3]
- high osmolarity; not recommended — Sports drinks, sodas, and full-strength juices have very high osmolarity (too much sugar, too little sodium) and are not ideal rehydration fluids in significant illness[4]
- 50-100 mL (under 2) / 100-200 mL (ages 2-10) per stool — Give ORS as frequent small sips after each loose stool[5]
- no urine 8+ hrs; can't keep fluids down; confusion — Red-flag dehydration signs warranting medical care include no urine for 8+ hours, inability to keep fluids down, and confusion or dizziness[6]
- [1]WHO/UNICEF, Oral Rehydration Salts: Production of the new ORS (WHO/FCH/CAH/06.1, 2006) — WHO/UNICEF. Oral Rehydration Salts: Production of the new ORS. WHO/FCH/CAH/06.1, 2006.
- [2]Hahn, Kim, Garner, BMJ 2001 — Hahn S, Kim Y, Garner P. Reduced osmolarity oral rehydration solution for treating dehydration caused by acute diarrhoea in children. BMJ 2001;323(7304):81-85.PMID: 11451782
- [3]Buccigrossi et al., Sci Rep 2020 — Buccigrossi V, et al. Functional and metabolic effects of oral rehydration solution on intestinal mucosa. Sci Rep 2020.PMID: 32385331DOI: 10.1038/s41598-020-64818-3
- [4]Chang JG, Am Fam Physician 2017 — Chang JG. Oral Rehydration Therapy. Am Fam Physician 2017;96(11):700-701.
- [5]Hartman et al., Am Fam Physician 2019 — Hartman S, et al. Dehydration: Diagnosis and Management. Am Fam Physician 2019;99(3):159-165.
- [6]Cleveland Clinic; CDC — Cleveland Clinic; U.S. Centers for Disease Control and Prevention. Dehydration: signs and when to seek care.
Medically reviewed: 2026-06-21. Every figure on this page is sourced to the named primary references above.
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Frequently Asked Questions
Why do doctors always say to drink fluids when you're sick?
Fluid intake during illness serves multiple purposes: it replaces fluids lost through fever, sweating, and nasal discharge; thins mucus to relieve congestion; supports immune cell transport through the lymphatic and circulatory systems; helps the body maintain fever (which fights infection); and aids medication absorption. It's one of the most effective supportive care measures for respiratory infections.
What is the best thing to drink when you have a cold or flu?
Warm fluids are most effective - a Rhinology journal study showed hot beverages improve nasal airflow better than cold drinks. Top choices include: warm water with honey and lemon, chicken broth or soup (provides electrolytes and anti-inflammatory compounds), herbal teas like ginger or chamomile, and oral rehydration solutions if you have fever or vomiting. Water at any temperature is always a good baseline.
How much water should I drink with the flu?
Adults with the flu should drink 3.0-4.0 liters (12-16 cups) of fluid per day, which is 1-2 liters above normal intake. If you have a fever, add approximately 100-150ml for every degree Celsius above normal. The key indicator is urine color - aim for pale yellow. If you have vomiting or diarrhea, use oral rehydration solutions to replace lost electrolytes.
Can you drink too much water when sick with a cold?
While uncommon, it is possible to overhydrate (hyponatremia) if you drink excessive amounts very rapidly, especially if you also have vomiting or diarrhea that depletes sodium. A 2012 BMJ study cautioned against 'pushing fluids' excessively during respiratory illness. Stick to the recommended 3-4 liters daily and include some electrolyte-containing beverages rather than exclusively plain water.
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