Several overlapping mechanisms explain why trips to the bathroom increase when you come down with the flu. Chills drive blood away from your skin and toward your core, signaling your kidneys to dump the extra central fluid. On top of that, you are likely drinking more than normal, your body is breaking down muscle protein at a faster rate during fever, and the medications in your cabinet can quietly alter how your bladder works. The picture is more layered than any single explanation, and a couple of the body’s own hormonal responses actually pull in opposite directions.
How Chills Push Fluid Toward Your Kidneys
When you get the chills, your body constricts blood vessels near the skin surface to conserve heat. That peripheral vasoconstriction squeezes blood inward, increasing the volume of blood circulating through your chest and core. Your kidneys read this shift as a signal that you have too much fluid on board, even though total body water hasn’t changed, and they respond by filtering out more of it as urine. Research on cold-induced vasoconstriction has shown that when peripheral blood vessels tighten and limb volume shrinks, sodium excretion rises in proportion to the expansion of central blood volume.1PubMed. Urinary responses to cold temperature during water immersion In practical terms, the same thing that makes your hands feel icy during a flu-related chill is pushing fluid toward your kidneys and out through your bladder.
This effect can repeat in waves. Flu fevers often cycle: your temperature climbs, you feel hot, then it drops and the chills return. Each round of shivering and vasoconstriction can trigger another small burst of urine production. If you’ve ever noticed that your need to pee seems to spike right when you’re huddled under blankets feeling coldest, this is why. The phenomenon is sometimes called cold diuresis, and it is well recognized outside of illness too, familiar to anyone who has spent time in frigid weather and noticed the same urge.
You Are Probably Drinking More Than Usual
The most straightforward reason for peeing more is also the easiest to overlook: you are taking in extra fluid. The universal advice to “drink plenty of fluids” when sick means most people with the flu are sipping water, juice, broth, or tea far more often than they would on a normal day. Your kidneys are efficient at matching output to input, so the more liquid goes in, the more comes out. This alone can double or triple your usual urination frequency without any exotic physiology at play.
What you drink matters too. Tea is one of the most popular sick-day beverages worldwide, and it contains caffeine, a mild diuretic. A review of the medicinal properties of tea noted that its consumption can have diuretic effects because of its caffeine content.2Food Research International. Medicinal and therapeutic potentialities of tea (Camellia sinensis L.) – A review Coffee, certain herbal teas, and even some commercial cold-and-flu drinks with added caffeine will do the same. If you are swapping plain water for several cups of caffeinated tea across a sick day, the caffeine adds a second push on top of the sheer volume of liquid. The diuretic effect of caffeine is modest in people who drink it regularly, but during illness you may be more sensitive to it, especially if you’ve been eating less and your stomach is empty.
Fever Burns Through Protein, and Your Kidneys Have to Clean Up
Fever is metabolically expensive. When your temperature rises, your body ramps up the breakdown of muscle protein to fuel the immune response and generate heat. A landmark study demonstrated that a signaling molecule released during fever stimulated net protein degradation in muscle tissue by roughly 60 to 120 percent compared to normal conditions.3New England Journal of Medicine. Stimulation of muscle protein degradation and prostaglandin E2 release by leukocytic pyrogen (interleukin-1) That protein breakdown produces waste products, particularly urea and other nitrogen-containing compounds, that must be dissolved in water and excreted by the kidneys.
Think of it as a higher solute load. When there is more metabolic waste to flush out, the kidneys need more water to carry it, and that means more urine. The same study found that at fever-level temperatures the effect was amplified even further, with protein breakdown increasing and the inflammatory molecule driving it still adding on top of what the heat alone was doing.3New England Journal of Medicine. Stimulation of muscle protein degradation and prostaglandin E2 release by leukocytic pyrogen (interleukin-1) This catabolic burst is part of why people with the flu often feel so weak and sore: their muscles are literally being dismantled to some degree. The kidneys’ cleanup of all that extra waste means more trips to the bathroom, even if fluid intake stays constant.
Inflammation Sends Competing Signals to the Kidneys
The flu triggers a surge of inflammatory signaling molecules called cytokines. These cytokines have real effects on the hormones that control how much water your kidneys hold onto or release, and the signals are surprisingly contradictory.
On the “pee more” side, preclinical evidence suggests that inflammation can stimulate the release of natriuretic peptides, hormones produced by the heart that promote the excretion of sodium and water. Researchers examining the link between inflammation and natriuretic peptide levels noted that nonhemodynamic triggers for their release exist, with several studies implicating inflammatory stimuli as a cause.4PubMed Central. Inflammation and Circulating Natriuretic Peptide Levels When natriuretic peptides go up, the kidneys let more sodium and water pass through into the urine. During a flu infection, where inflammation is systemic, this could contribute to increased urine volume.
On the “hold onto water” side, the same inflammatory cytokines, particularly interleukin-6, can trigger the release of vasopressin, also known as antidiuretic hormone. Vasopressin tells the kidneys to reabsorb water rather than excrete it. A review of the roles of interleukin-6 in disease found that elevated levels of this cytokine have been seen in severe inflammatory and infectious states associated with inappropriate vasopressin secretion.5PubMed. The pathophysiologic roles of interleukin-6 in human disease Research on inflammation-driven hormonal changes has confirmed that during infection, proinflammatory cytokines including interleukin-6 can independently stimulate the brain’s hypothalamus and pituitary gland to release vasopressin, which acts to retain water.6PubMed Central. Mild encephalitis/encephalopathy with a reversible splenial lesion secondary to encephalitis complicated by hyponatremia
So inflammation simultaneously nudges the kidneys toward producing more urine (via natriuretic peptides) and toward retaining water (via vasopressin). In most mild to moderate flu cases, the diuretic forces win out: you are drinking extra fluids, chills are shunting blood centrally, and metabolic waste is rising. But in severe infections, the vasopressin side can dominate, sometimes leading to water retention, diluted blood sodium, and reduced urine output. This is why critically ill patients with influenza occasionally develop the opposite problem, dangerously low sodium from too much water retention, rather than excessive urination.
Cold and Flu Medications Can Change Bladder Behavior
The pills and syrups people reach for during the flu can independently affect how often and how easily they urinate. Pseudoephedrine, the decongestant found in many over-the-counter cold products, works by tightening blood vessels to reduce nasal swelling. But it does not limit that effect to the nose. It activates receptors throughout the body, including in the bladder and urethra.
A study that tracked urinary symptoms in patients taking pseudoephedrine for nasal congestion found that people aged 50 and older experienced a measurable worsening of both storage and voiding symptoms after taking the drug. Their storage symptom scores, which include urinary frequency and urgency, rose from about 4.6 to 5.4 on a standard scale, while voiding symptom scores also increased.7PubMed Central. Voiding dysfunction in patients with nasal congestion treated with pseudoephedrine: a prospective study In younger adults the effect was not significant. For older people, though, a decongestant taken to breathe easier can quietly make the bladder more irritable and harder to empty completely, a combination that sends you to the bathroom more often with a frustrating sense that you did not fully void.
Other common flu medications carry their own urinary quirks. Antihistamines like diphenhydramine (found in nighttime cold formulas) can relax the bladder muscle and make it harder to initiate urination, leading to incomplete emptying and a frequent urge to go again. Ibuprofen and acetaminophen, while they do not directly affect urination, require kidney processing and add to the overall workload on the renal system when taken alongside high fluid intake. If you are stacking multiple over-the-counter products, as many people do during a bad flu, the cumulative bladder effects can be more noticeable than any single drug alone.
Stress, the Nervous System, and a Twitchy Bladder
Being sick is stressful, both psychologically and physiologically. The flu activates your sympathetic nervous system, the “fight or flight” branch, as part of the broader immune response. That heightened sympathetic tone does not just speed up your heart rate or make you feel on edge. It also affects the bladder. Research has shown that enhanced sympathetic signaling and neuroinflammatory changes within the bladder wall can heighten bladder sensitivity and amplify urinary symptoms, including shortened intervals between voids and overactivity of the bladder muscle.8PubMed Central. Recurrent urinary tract infections and psychological burden: mechanisms and integrative perspectives
In the context of the flu, this means your bladder may be sending “time to go” signals at lower fill volumes than normal. You are not necessarily producing dramatically more urine in this case; instead, your bladder is simply less tolerant of being full. Combined with the actual increase in urine volume from the mechanisms already described, a more sensitive bladder amplifies the experience of needing to pee constantly. People who already deal with an overactive bladder or stress-related urinary urgency tend to notice this more acutely when they get sick.
When Frequent Urination Points to Something Else
Most of the time, peeing a lot during the flu is a harmless byproduct of the factors above. But occasionally, frequent or painful urination during a viral illness is a sign that something additional is going on.
Urinary tract infections can develop alongside or shortly after influenza. A case report documented cystitis, a bladder infection, in an otherwise healthy adult following an influenza B infection. Investigators noted that the relationship between influenza and urinary bacteriuria is not fully understood, and in at least one pediatric case it was unclear whether the bacteria found in the urine were responsible for the patient’s symptoms or appeared as a consequence of the viral infection itself.9PubMed Central. Uncomplicated Cystitis in an Adult Male Following Influenza B Virus Infection The takeaway is that burning, pain, or blood during urination should not be written off as just part of having the flu. Those are symptoms of a possible bacterial infection that may need separate treatment.
Severely reduced urine output is actually the more dangerous urinary change to watch for during the flu. Dehydration from vomiting, diarrhea, or simply not drinking enough can drop urine production to very low levels. And as described earlier, severe inflammation can trigger excessive vasopressin release, causing the body to retain water and dilute blood sodium to dangerously low concentrations. If you notice that you have stopped urinating much at all, that your urine is extremely dark, or that you feel confused or unusually drowsy, those warrant medical attention rather than another cup of tea.
Why It Often Feels Worse at Night
Many people report that the constant need to urinate during the flu is most disruptive overnight, and a few factors converge to explain this. Lying down redistributes fluid from your legs into your core and kidneys, mimicking the central volume shift caused by vasoconstriction. This is the same reason people with heart failure or leg swelling pee more at night. When you are already dealing with flu-driven fluid shifts, lying flat amplifies the effect.
Fever also tends to spike in the evening and overnight, which intensifies the chill-vasoconstriction-diuresis cycle right when you are trying to sleep. And if you took a nighttime cold formula containing both a decongestant and an antihistamine, you may be dealing with the combined bladder effects of those drugs at the same time. The result is a particularly frustrating loop: you feel exhausted and need sleep, but your bladder keeps pulling you out of bed. Reducing fluid intake in the two hours before sleep, while still staying well hydrated during the day, and choosing medications that do not contain pseudoephedrine at bedtime can help somewhat.
The Caffeine and Hydration Trade-Off
There is an irony in the way people typically manage flu hydration. The beverages considered most comforting during illness, particularly tea and coffee, are the same ones that promote additional urine production through their caffeine content.2Food Research International. Medicinal and therapeutic potentialities of tea (Camellia sinensis L.) – A review You drink them to stay hydrated and soothe a sore throat, but they simultaneously pull fluid through the kidneys faster than plain water would. This does not mean tea is a bad choice when sick. The net fluid balance is still positive; you absorb more from a cup of tea than you lose from the caffeine’s diuretic nudge. But if frequent urination is itself becoming a source of misery, especially at night, swapping caffeinated tea for herbal varieties, warm water with honey, or broth can reduce the diuretic effect without sacrificing the warmth or fluids your body needs.
Broth, incidentally, offers something caffeine-free beverages and plain water do not: sodium. Because flu-driven inflammation can push natriuretic peptides higher and because sweating during fever flushes salt out of the body, your sodium stores take a hit during illness. Broth or an electrolyte drink replaces some of that salt, which helps the kidneys recalibrate and may modestly reduce the volume of dilute urine they produce. It is not a dramatic fix, but for someone exhausted by constant bathroom trips, the difference between water and a salty broth can be noticeable over the course of a day.