Can Flu Increase Blood Pressure? The Drop Matters More

What this guide covers

Illness pushes blood pressure both ways. Upward causes come first below, downward ones second.

The short answer, in both directions

Influenza raises blood pressure in plenty of people while they are acutely unwell. Fever speeds the heart, pain and poor sleep switch on the stress response, coughing spikes pressure in short bursts, and the cold and flu products on the nightstand contain drugs designed to constrict blood vessels. Stack those and a person who normally sits at 122/76 can read 140/88 on day two of the flu with nothing sinister going on. Understanding what generates the pressure in your arteries makes the pattern obvious: heart rate up, vessels tightened, pressure up.

Then there is the other half of the answer, and it is the half that saves lives. When an infection becomes severe, the body floods with inflammatory signals that relax the walls of small arteries. Resistance collapses. Blood pools in the periphery. The heart races to compensate and, for a while, it succeeds, so the reading looks acceptable. Then it stops succeeding, and the pressure falls. That falling reading during infection is called septic shock once it crosses certain thresholds, and it is a time-critical emergency in the same category as a heart attack.

So illness moves the reading in whichever direction the illness itself is pushing. Mild illness pushes up. Severe illness pushes down. A person who only knows to fear high numbers will miss the more dangerous event entirely, and that is the single most useful idea here.

Two of those four rows point downward, and they are where the acute risk sits. If you own a monitor because you worry about hypertension, you already own the tool that detects the more urgent problem. You just have to know that a low reading in a sick person means something. That is a different reflex from the one the usual story about why pressure runs high trains into you.

Why illness moves the number at all

Four separate mechanisms are at work whenever you are unwell, and they do not all pull the same way. Being able to name which one is dominant tells you what your reading means.

Put those four together and you can predict most of what happens. A short, painful, feverish illness with normal drinking reads high. The same illness with two days of poor fluid intake reads lower than usual. A severe bacterial infection reads low whatever the fever is doing, because the inflammatory mechanism overwhelms the others.

Colds, flu, sinus infections and allergies

These four get grouped together in search because they feel similar. They behave differently.

A common cold is a mild upper airway infection with little systemic inflammation. Left alone, without medication, it barely moves blood pressure. What people notice at home is usually broken sleep, mouth breathing and a couple of restless nights. A reading up four or five points during a head cold sits inside the ordinary noise of any week of regular home monitoring.

Influenza is a different animal because it is systemic. Fever of 101 to 103 degrees Fahrenheit, deep muscle pain, headache and days of broken sleep add up, and a rise of five to fifteen points on the top number during the worst two days is unremarkable. The more clinically relevant fact is that influenza infection carries a measurably increased risk of heart attack in the weeks that follow, driven by inflammation and clotting tendency. That is one reason vaccination is recommended for people with existing heart disease.

Sinus infections raise pressure through pain. Facial pressure, a pounding head and tenderness on leaning forward drive the same sympathetic response as any other pain. If the reading is up during sinusitis and the pain settles with treatment, the number settles with it. Ear infections appear in the same searches for the same reason, and neither produces sustained hypertension.

Seasonal allergies are the interesting case, because allergic rhinitis on its own does very little to blood pressure. Sneezing and itchy eyes do not raise it. Poor sleep from a blocked nose can nudge it, the same way any run of bad nights nudges it. What moves the number in allergy season is the treatment: oral decongestants, and to a lesser degree some combination products. Antihistamines alone are generally neutral. When someone says their allergies raise their pressure every spring, the pills are the likelier explanation than the pollen.

One severe exception belongs here. Anaphylaxis causes massive vasodilation and a rapid, dangerous fall in blood pressure alongside swelling and breathing difficulty. That is the far end of the scale from hay fever and needs emergency treatment with epinephrine.

When infection sends it down: the signal that matters

Everything above concerns illness that is behaving itself. This section concerns illness that is not.

When bacteria or their fragments reach the bloodstream in quantity, the immune response becomes systemic. Nitric oxide and inflammatory mediators flood the circulation and relax the smooth muscle wrapped around small arteries, so systemic vascular resistance drops sharply. Capillaries turn leaky at the same time and plasma escapes into the tissues, so circulating volume falls too. Both halves of the pressure equation are attacked at once.

The body fights back well at first. The heart speeds up, vessels in the gut and skin clamp down to preserve flow to the brain and kidneys, and the reading stays inside the normal range. This is compensated shock, and it is the dangerous phase, because the monitor shows a number that looks fine while the patient deteriorates. The clues are a fast pulse, fast breathing, and a person who does not seem right. Then compensation fails and the reading falls off a cliff.

Sepsis kills through inadequate organ perfusion, and perfusion depends on mean arterial pressure. Critical care teams resuscitate toward a target of at least 65 mmHg for exactly that reason, because below roughly that level the kidneys, brain and gut stop receiving enough flow. You can compute the same figure at home with the mean arterial pressure calculator, and in a sick person it carries more information than the systolic reading alone.

Work an example. A reading of 96/54 looks unremarkable to someone who has spent years worrying about high numbers. Its mean arterial pressure is 54 plus 42 divided by 3, which is 68: a hair above the resuscitation target, in a person whose usual 128/80 gave a mean of 96. A fall of 28 points of mean pressure in a feverish adult is a red flag however benign 96/54 looks in isolation. The method for working it out by hand takes ten seconds and is easier to learn now than in a crisis.

Two details make sepsis easy to miss at home. The person may have no fever at all, since older adults and the immunosuppressed often run a normal temperature while severely septic. And baseline matters: someone whose usual reading is 158/90 can be in serious trouble at 118/70, numbers that would earn congratulations in any other context.

COVID: the short story and the unsettled one

COVID deserves separate treatment because the evidence sits in two different places.

Acutely, COVID behaves like other significant respiratory infections. Mild cases with fever and aching push the reading up modestly. Severe cases produce sepsis physiology and the reading falls, with hypotension among the features that land people in intensive care. The severity spectrum is simply wider than with a common cold, so both ends of the range show up more often.

Afterwards the picture is still forming, and the uncertainty is worth stating plainly. Several large observational studies report higher rates of new hypertension diagnoses in the months following COVID infection compared with matched uninfected people, with a larger effect after severe illness. Other work found blood pressure control worsening across the pandemic period at a population level, which is hard to separate from lockdown weight gain, reduced exercise and missed routine care. A real biological effect probably exists and its size is not yet pinned down.

A clearer post-viral phenomenon gets confused with hypertension. Some people develop dysautonomia after COVID, including postural orthostatic tachycardia syndrome, where standing produces a large sustained rise in heart rate. Pressure in that condition is often normal or low while the pulse is dramatic, and the sensation is pounding, dizziness and exhaustion on standing. People assume their blood pressure has gone haywire when the pressure may be the one thing behaving. Untangling them means measuring lying and standing, and the distinction between the two numbers is set out in the piece on whether pulse and pressure move together.

Anemia, blood loss and feeling cold

The question “can anemia cause high blood pressure” comes up often, and the direct answer is that it usually does the reverse.

Anemia means fewer red cells carrying oxygen. The body compensates by pushing more blood around per minute, so cardiac output rises and the heart beats faster. Blood also becomes less viscous, lowering resistance to flow. The typical picture in significant anemia is a normal or slightly low pressure with a raised pulse, a wide pulse pressure, pallor, breathlessness on exertion and tiredness.

Where the two appear together, something else usually links them. Chronic kidney disease causes both anemia and high blood pressure, so they travel as companions without one causing the other. Erythropoietin-stimulating agents used for renal anemia do raise pressure, which is recognized and monitored for. If you have both findings, ask what condition produces them together.

Acute blood loss is the sharper version of the same physiology. A young, fit adult can lose a substantial fraction of their blood volume while holding an almost normal reading, propped up by a racing pulse and clamped-down peripheral vessels. When that reading finally drops it drops fast, so trauma teams watch pulse and skin perfusion as closely as they watch the cuff. Slow chronic loss, from heavy periods or a bleeding ulcer, produces the anemia picture instead.

Does low blood pressure make you feel cold? It can. When pressure is low the body prioritizes the brain, heart and kidneys and constricts vessels supplying the skin, so less warm blood reaches your hands and feet and they feel cold, sometimes clammy or pale. Anemia adds its own version of the same sensation. Someone with low readings and permanently cold hands is describing a coherent picture, and the article on the signs that go with low pressure covers the rest. The reverse question has no established mechanism, though beta blockers commonly cause cold hands and feet, making the drug the likelier link than the pressure.

Surgery, anesthesia and the dentist’s chair

Medical procedures produce some of the largest short-term swings a person will ever record, in both directions, and almost none of it reflects underlying pressure.

Before surgery the reading is usually high. Anxiety, an unfamiliar environment, fasting, a full bladder and a nurse with a cuff produce a textbook stress response, and anesthetists know pre-operative readings are poor estimates of a patient’s true pressure. What they care about is whether it is high enough to raise perioperative risk. Readings around 180 systolic or 110 diastolic and above are the ones that prompt a conversation about postponing an elective operation, and even then it depends on the surgery. A reading of 150/88 on the morning of a hernia repair rarely changes anything.

During anesthesia the pressure typically falls. Induction agents cause vasodilation and reduce cardiac contractility, so a drop after the patient goes under is expected and is managed continuously with fluids and drugs. Intubation and surgical incision each produce a spike in the opposite direction from intense sympathetic stimulation. The anesthetist spends the whole operation flattening those swings, so a patient worried about their blood pressure under anesthesia is worrying about the one variable being watched second by second.

After surgery the number often runs high for a while. Pain leads the list, followed by the stress hormone response to tissue injury, fluid given during the operation, urinary retention, and regular tablets held before the procedure. Post-operative hypertension of this kind usually settles within hours to a couple of days. When it persists for weeks, assume something has changed: pain still uncontrolled, medication not restarted correctly, or an underlying hypertension that was masked before. That is a conversation for the surgical team.

Dental procedures combine two effects. Dental anxiety is common and produces large readings in the chair. On top of that, most dental local anesthetic contains a small amount of epinephrine to prolong the numbing and reduce bleeding, which can briefly lift heart rate and pressure, particularly if some enters a vessel. The quantities are low and the effect in a healthy person is minor. Dentists check blood pressure before certain procedures because they want to know whether it is safe to proceed, and a very high reading means postponing and referring. If yours reads high at the dentist and normal at home, the chair is the outlier and your home series is the better data, in the same way that a single office reading rarely tells the whole story.

Is a reading taken while you are sick worth acting on?

Mostly no, with two exceptions important enough to justify the section.

The general rule first. One elevated reading during acute illness measures your usual blood pressure poorly, and no diagnosis should be made from it. Hypertension is diagnosed from repeated readings taken under standard conditions when you are well. If day three of the flu gives you 148/92, write it down with the word “flu” beside it and take a proper series two weeks after recovery. Judging yourself on that basis is like weighing yourself in a winter coat. The right conditions are laid out in the guide on timing your checks, and the equipment side in the monitor comparison.

Illness also degrades the measurement itself. Shivering ruins an oscillometric reading, as does coughing during inflation or a fast irregular pulse. If the machine gives you three wildly different answers in five minutes, it is reporting the conditions accurately. Basics still matter: the same arm every time, feet flat, back supported, and a cuff that fits your arm. A wrist device or a watch is a poor choice here, for reasons covered in the piece on wearables.

Now the two exceptions, and they both point downward or upward far enough to matter.

Between those two extremes, a reading during illness is data about the illness. Record it, note the circumstances, and judge your blood pressure later from a proper series. Whether that series lands somewhere comfortable or in the range where 130/80 starts to count as stage 1 is a question for a well week.

When to worry, in exact numbers

Two thresholds matter during illness, one at each end.

At the other end, use your own baseline. A systolic figure at or below 100 in an infected adult who normally runs higher is a warning, and it becomes an emergency alongside the sepsis signs listed earlier. A mean arterial pressure below 65 mmHg in a sick person is an emergency number in any hospital in the world.

Some symptoms override the reading entirely. Face drooping, arm weakness or speech difficulty means stroke until proven otherwise, and the action is to call emergency services immediately whatever the cuff says. The full framing is in the article on the pressures associated with stroke. Chest pain, breathlessness at rest and collapse sit in the same category, where no reading is ever the deciding factor.

Two groups need a lower threshold for seeking help during any illness. Older adults, because infection presents atypically and compensation reserves are smaller. And anyone pregnant, because pregnancy changes the rules in both directions: fever and infection need prompt assessment, and a reading of 140/90 or above in pregnancy needs same-day contact with a maternity provider whatever else is going on, as explained in the pregnancy article.

Everyone else can use a simpler rule. A modestly raised reading with no worrying symptoms is expected during a short illness and needs no action beyond noting it. A falling reading, or any reading paired with confusion, breathlessness, chest pain or reduced urine output, needs a phone call now.

Questions people ask