There is no single number of glasses or liters that applies to everyone who is sick, because the amount of fluid you lose and the type of illness you have vary enormously. A mild cold and a bout of severe food poisoning place very different demands on your body. What the evidence does show is that most common illnesses increase fluid losses in ways you may not notice, and that the familiar advice to “drink plenty of fluids” is well-intentioned but surprisingly light on hard data. Getting hydration right when you are sick means understanding where the extra losses come from, watching for signs that you are falling behind, and knowing when more fluid can actually do harm.
Why Being Sick Drains More Fluid Than You Think
The most obvious culprits are vomiting and diarrhea, but fever alone creates a significant hidden drain. When your body raises its thermostat to fight off an infection, several things happen at once. Blood gets shunted away from your skin’s surface to conserve heat, and your metabolism shifts to burning fat and protein instead of relying on circulating glucose. More relevant to hydration, fever reduces production of antidiuretic hormone, which is the signal that tells your kidneys to hold on to water. With less of that hormone circulating, you urinate more than usual even though you are losing extra fluid through sweat and faster breathing.1PubMed Central. Primary Care Febrile Illness in the Athlete – Section: Thermoregulation, Metabolism, and Fluid Homeostasis For every degree Celsius your temperature climbs above normal, your insensible water loss (the fluid that evaporates from your skin and lungs without you sweating visibly) goes up as well.
Respiratory infections add another layer. Your airways are lined with a thin layer of mucus that depends on fluid transport across the tissue to stay properly hydrated. When that mucus dries out, it thickens, cilia slow down, and your body’s first-line defense against trapped pathogens weakens.2PubMed Central. Physiology and pathophysiology of human airway mucus Staying adequately hydrated helps keep airway secretions thin enough for your cilia to sweep debris out, which is one of the reasons you feel more congested when you are dehydrated.
Your Thirst Signal May Lie to You
You might assume your body would tell you when it needs more water, and under normal conditions, thirst is a reasonably good guide. But illness can interfere with that signal. Animal research shows that inflammatory molecules released during infection, particularly cytokines like IL-1β and TNF-α, act on areas of the brain involved in thirst and sodium appetite, dampening both.3Appetite. The gut-brain axis and sodium appetite: Can inflammation-related signaling influence the control of sodium intake?4PubMed Central. Participation of α2-adrenoceptors in sodium appetite inhibition during sickness behaviour following administration of lipopolysaccharide In plain terms, the same immune response that makes you feel awful may also turn down the internal alarm that would normally drive you to reach for a glass of water. This is part of a broader set of behaviors researchers call sickness behavior: the fatigue, appetite loss, and social withdrawal that characterize being ill. Reduced thirst fits into that pattern, but it does not mean your body needs less fluid. It means you may need to drink deliberately rather than waiting until you feel thirsty.
Stomach Bugs Need a Different Approach Than Colds
If you have gastroenteritis (the stomach flu), the primary danger is losing not just water but also electrolytes, especially sodium and potassium, through vomiting and diarrhea. Plain water replaces volume but not the salts your cells need to function. Oral rehydration solutions, which contain a specific balance of sugar and salts, are the gold standard in this situation. For moderate dehydration with roughly 3 to 9 percent body-weight loss, clinical guidelines call for about 50 to 100 milliliters of oral rehydration solution per kilogram of body weight over four hours, followed by a maintenance phase of resumed eating and continued sipping.5PubMed Central. Understanding the use of oral rehydration therapy: A narrative review from clinical practice to main recommendations For a person weighing around 70 kilograms, that works out to roughly 3.5 to 7 liters over the initial four-hour rehydration window, though in practice you would sip steadily rather than trying to drink it all at once.
For respiratory illnesses like colds and the flu, the math is less precise. The widely repeated advice to “push fluids” is based more on tradition than on rigorous trials. A systematic review found no direct published evidence for the optimal amount or type of fluid to drink during a fever.6PLOS ONE. Symptomatic fever management in children: A systematic review of national and international guidelines – Section: Discussion That does not mean fluids are unimportant. It means nobody has run the kind of controlled trial that would let doctors say “drink exactly X liters per day when you have the flu.” The practical takeaway: aim for enough to keep your urine a pale straw color, replace the extra losses from fever and faster breathing, and do not force enormous volumes just because a well-meaning relative insists.
When Drinking Too Much Becomes Dangerous
This is the part most people do not hear. Overzealous fluid intake during illness can cause a condition called hyponatremia, where the sodium concentration in your blood drops too low. The risk is real, not just theoretical. Many infections trigger increased secretion of antidiuretic hormone (sometimes called SIADH when the secretion is inappropriate), which causes your kidneys to retain water. If you are simultaneously drinking large amounts of fluid, you dilute your blood sodium further. The combination of excess water and hormonally driven water retention can lower sodium to dangerous levels, causing confusion, seizures, and in rare cases, death.7Journal of Infection. Hyponatremia in patients with infectious diseases – Section: Summary
This is especially relevant for illnesses like pneumonia, meningitis, and severe respiratory infections. In these cases, the body’s hormonal response to infection actively works against excess fluid intake. The old advice to “drink as much as you can” is not just unsupported; it can worsen outcomes. A sensible approach is to drink enough to stay hydrated and replace losses, but not to force fluid beyond what your body is asking for once basic hydration is covered.
How to Check Whether You Are Drinking Enough
Urine color remains one of the simplest and most reliable self-checks. Pale yellow to straw-colored urine generally indicates adequate hydration. Dark yellow or amber urine suggests you need to drink more. A study of hospitalized patients with dengue fever found strong correlations between urine color and direct measures of hydration: the darker the urine, the higher the concentration of waste products and the more dehydrated the person was.8PLOS Neglected Tropical Diseases. Assessing dehydration status in dengue patients using urine colourimetry and mobile phone technology – Section: Results You do not need a phone app or color chart to use this in practice: if your urine looks like apple juice, drink more; if it looks like lemonade, you are probably fine.
Other signs of dehydration worth watching for include a dry mouth, headache that worsens when you stand up, dizziness, and producing very little urine at all. In children and infants, sunken eyes, no tears when crying, and a sunken soft spot on the top of the head are warning signs. By the time these physical signs are obvious, dehydration is already moderate, so catching it early by keeping an eye on urine color is better than waiting for clinical symptoms.
Hot Drinks Do More Than You Would Expect
There is a real and measured benefit to warm fluids beyond just adding hydration. A controlled study compared a hot fruit-flavored drink to the same drink served at room temperature in people with cold and flu symptoms. The hot version produced immediate and sustained relief from runny nose, cough, sneezing, sore throat, chills, and tiredness, while the room-temperature version only helped with runny nose, cough, and sneezing.9PubMed. The effects of a hot drink on nasal airflow and symptoms of common cold and flu Interestingly, the hot drink did not actually change objective measurements of nasal airflow. The nose was no more open, but people felt like it was, and they felt less miserable across almost every symptom category. This makes hot tea, broth, and soup genuinely therapeutic for comfort, even if the mechanism is partly perceptual. It also means the specific choice of warm liquid matters less than the temperature: hot water with lemon, chicken soup, and herbal tea all do roughly the same job.
Plain Water vs. Electrolyte Drinks
For a garden-variety cold or flu, plain water is perfectly adequate. The marketing of electrolyte beverages and sports drinks can make people feel like they need something fancier, but the evidence for switching to electrolyte solutions is limited to situations involving substantial fluid loss through sweat, vomiting, or diarrhea. Two separate studies comparing plain water against electrolyte-containing beverages for rehydration after exercise-induced dehydration found no meaningful difference in how much body weight people regained, how much fluid they retained, or how their blood sodium and overall hydration markers looked.10PubMed Central. Comparison of coconut water and a carbohydrate-electrolyte sport drink on measures of hydration and physical performance in exercise-trained men – Section: Results11Journal of Physiological Anthropology and Applied Human Science. Rehydration after Exercise with Fresh Young Coconut Water, Carbohydrate-Electrolyte Beverage and Plain Water The exception, as discussed earlier, is gastroenteritis, where oral rehydration solutions containing sodium and glucose are clearly superior because they exploit a specific absorption pathway in the gut that pulls both water and salt across together.
If you are sick with a respiratory illness and can eat even small amounts of food, you are likely getting enough sodium and potassium from crackers, broth, or fruit to maintain electrolyte balance. Save the specialty rehydration products for when you are losing significant volumes through your gastrointestinal tract.
Children and Older Adults Face Higher Stakes
Pediatric fluid management during illness deserves its own attention because children dehydrate faster than adults: they have a higher surface-area-to-body-mass ratio, which means relatively more insensible fluid loss, and their kidneys are less efficient at concentrating urine. A French national survey of healthcare professionals found that oral hydration was the most commonly recommended physical treatment for fever in young children, suggested for about 39 percent of feverish pediatric patients overall, with higher rates for infants and toddlers (around 42 to 44 percent) than for older children.12PLOS ONE. Symptomatic Management of Fever in Children: A National Survey of Healthcare Professionals’ Practices in France – Section: Results Yet despite the universality of this recommendation, a systematic review of international guidelines concluded that no direct evidence exists for the optimal amount or type of fluid during pediatric fever, and cautioned that blindly increasing fluids could cause harm in some contexts.6PLOS ONE. Symptomatic fever management in children: A systematic review of national and international guidelines – Section: Discussion
Older adults face a different problem. Even in healthy conditions, people over 65 show a blunted thirst response to dehydration. After water deprivation, healthy elderly men experienced greater increases in blood concentration markers than younger men but reported being less thirsty and drank less afterward, failing to fully restore their fluid balance in the same time frame.13PubMed. Reduced thirst after water deprivation in healthy elderly men Even under stresses like heat or exercise, older adults drink less and take longer to restore their fluid levels.14PubMed. Influence of age on thirst and fluid intake Layering an illness on top of this already-dampened thirst mechanism makes older adults especially vulnerable to dehydration. A caregiver may need to offer fluids regularly, because the older person simply will not feel thirsty enough to drink on their own.
The Ibuprofen and Dehydration Combination
One practical risk that comes up surprisingly often involves common fever-reducing medications. Ibuprofen and similar nonsteroidal anti-inflammatory drugs reduce blood flow to the kidneys, which is usually harmless when you are well-hydrated. But when dehydration is already present, that reduced blood flow can tip the kidneys into acute injury. A study of children with gastroenteritis found that those who received ibuprofen while dehydrated were significantly more likely to develop acute kidney injury: after adjusting for how dehydrated the children were, ibuprofen exposure roughly doubled the odds of kidney impairment.15PubMed. Ibuprofen-associated acute kidney injury in dehydrated children with acute gastroenteritis – Section: RESULTS This does not mean ibuprofen is dangerous in general. It means that if you or your child has a stomach bug with vomiting and diarrhea, getting fluid intake up before reaching for ibuprofen is important. Acetaminophen (paracetamol) does not carry the same kidney risk and may be the safer choice for fever reduction while dehydration is being corrected.
Dry Indoor Air Makes Things Worse
Environmental conditions affect your fluid needs more than most people realize, and this becomes particularly important when you are sick indoors during winter with the heating running. Dry air accelerates water loss from your airways. Experiments using human airway cells found that as relative humidity dropped from 95 percent to 60 percent to 30 percent, the mucus layer thinned by about 5, 35, and 58 percent respectively. The stressed cells also ramped up production of inflammatory signals like TNF-α and IL-6.16Communications Earth & Environment. Global warming risks dehydrating and inflaming human airways – Section: Results In a heated room during winter, indoor humidity can easily drop to 20 or 30 percent. If you are already fighting a respiratory infection, that dry air is compounding airway inflammation and increasing the rate at which fluid evaporates from your lungs with each breath. Using a humidifier or placing a bowl of water near a heat source can help, and drinking warm fluids serves double duty by hydrating both your body and your upper airways.
When More Fluid Is the Wrong Answer
Not every sick person benefits from extra hydration. In severe infections requiring hospitalization, particularly sepsis, aggressive fluid administration is a carefully calibrated clinical decision rather than an instinct to follow at home. Among patients treated in intensive care for severe sepsis, about two-thirds showed signs of fluid overload by the first day, and those with persistent fluid overload had roughly double the odds of dying in the hospital compared to those without it.17PubMed Central. Fluid Overload in Patients with Severe Sepsis and Septic Shock Treated with Early-Goal Directed Therapy is Associated with Increased Acute Need for Fluid-Related Medical Interventions and Hospital Death – Section: Results That scenario involves intravenous fluids in a hospital, not someone sipping tea at home, but it illustrates an important principle: fluid is medicine, and like any medicine, the dose matters.
People with congestive heart failure face a chronic version of this dilemma. The heart’s inability to pump efficiently means extra fluid can accumulate in the lungs and legs. Clinicians have long debated the right balance: traditional practice restricts fluid to about 1,000 to 1,500 milliliters daily, though some research has questioned whether strict restriction improves outcomes or merely makes patients miserable. If you have heart failure and get sick with a cold or stomach bug, the instinct to “push fluids” runs directly against your usual fluid limits. This is a case where you should call your doctor rather than guessing, because the right answer depends on your specific cardiac function, medications, and how well your kidneys are handling the extra load.
Evolutionary Reasons Your Body May Resist Drinking
It may seem counterintuitive that your body would suppress thirst during illness precisely when it needs more fluid. One hypothesis frames this as part of a broader adaptive program. Hydration and sleep have been proposed as behaviors that promote certain immune pathways, specifically vagal tone and the type of immune response that fights viral infections.18PubMed. “Starve a fever and feed a cold”: feeding and anorexia may be adaptive behavioral modulators of autonomic and T helper balance Yet the sickness behavior that includes reduced appetite and thirst may have evolved to serve a different function entirely: keeping a sick animal still and conserving energy. In that evolutionary context, not seeking water meant not moving, and not moving meant not exposing a weakened animal to predators. The trouble is that this ancient programming can work against you when you are lying safely on a couch with clean water within arm’s reach. Knowing that your body’s instincts may not serve your hydration needs during illness is a useful nudge toward intentional drinking rather than waiting until the urge strikes.