What Is a Bradykinin Cough and What Causes It?

A bradykinin cough is a persistent, dry cough triggered by the accumulation of a small protein fragment called bradykinin in the airways. It is best known as the most common side effect of a widely prescribed class of blood pressure medications, ACE inhibitors, affecting somewhere between 5 and 35 percent of people who take them. The cough can appear within hours of the first dose or not surface for months, which is part of what makes it so frustrating to pin down.

How Bradykinin Builds Up in the First Place

ACE inhibitors work by blocking an enzyme called angiotensin-converting enzyme. That enzyme has two main jobs: it converts a hormone precursor into angiotensin II, which raises blood pressure, and it breaks down bradykinin, a peptide the body naturally produces as part of its inflammatory and pain-signaling toolkit. Blocking the enzyme accomplishes the therapeutic goal of lowering blood pressure, but it also means bradykinin does not get cleared as efficiently. The peptide accumulates in the blood and, critically, in the tissues lining the airways.

Bradykinin is normally present in the lungs at low levels. When concentrations rise, the peptide binds to receptors on sensory nerve endings embedded in the airway walls. Research in animal models showed decades ago that bradykinin injected into the pulmonary circulation could increase activity in vagal C-fiber nerve endings by roughly fifteen-fold on average, confirming that these nerve fibers are the pathway through which bradykinin provokes coughing and other reflex responses in the lung.¹ Those C-fibers relay signals to the brainstem’s cough center, and when they fire persistently, you get a cough that does not go away on its own as long as the drug keeps bradykinin levels elevated.

The Receptor and Channel Pathway Behind the Cough Reflex

Most of bradykinin’s cough-provoking effects run through a specific receptor known as the B2 receptor. In experiments using a selective B2 receptor blocker, the cough-enhancing effect of bradykinin was reduced by about 70 to 80 percent, which tells researchers that B2 activation is the dominant trigger.² Once B2 receptors are activated, they set off a cascade involving inflammatory enzymes that produce prostaglandins and other mediators. These mediators, in turn, sensitize ion channels on the same nerve endings, lowering the threshold at which those nerves fire. The result is a cough reflex that triggers more easily and more often than it should, even in response to stimuli that would not normally make you cough.

This sensitization explains a hallmark of bradykinin cough: it often feels like a tickle or irritation deep in the throat or chest that you simply cannot suppress. The nerve endings are not responding to an infection or foreign object. They are responding to the body’s own inflammatory signaling molecules running higher than usual. That is why antibiotics, decongestants, and standard cough suppressants tend to do nothing for it.

What a Bradykinin Cough Actually Feels Like

Clinically, the cough is described as dry, tickly, and often bothersome enough to interfere with sleep or daily activities.³ It produces no mucus or phlegm, which distinguishes it from productive coughs caused by respiratory infections or chronic bronchitis. People often describe it as a persistent scratchy sensation that provokes bouts of coughing, particularly when lying down or during the night.

One of the trickiest aspects is its timing. The cough can start within hours of the first ACE inhibitor dose, but in many cases it creeps in weeks or even months into therapy.⁴ That delayed onset means both patients and doctors may not connect the cough to the medication. It gets blamed on seasonal allergies, a lingering cold, or acid reflux. In one study of patients on enalapril, about one in ten eventually had to stop the drug because the cough became intractable, and in nearly all of those cases the cough resolved within two weeks of discontinuation.⁵ Other reports note that the cough usually clears within a few days once the ACE inhibitor is stopped.³

The reversibility is actually one of the most useful diagnostic clues. Because there is no lab test or imaging study that confirms bradykinin cough, the diagnosis is largely one of exclusion combined with a drug-withdrawal trial. If the cough goes away after stopping the ACE inhibitor and comes back when the drug is restarted, that is about as close to a definitive answer as you will get.

Why Some People Get It and Others Do Not

If bradykinin accumulation happens in everyone who takes an ACE inhibitor, why does only a fraction develop a cough? Part of the answer is genetic. A meta-analysis looking at gene variants in the ACE gene and the bradykinin B2 receptor gene found that certain polymorphisms significantly raised the risk of developing the cough, but the effect was most pronounced in people of East Asian ancestry. In that population, a variant in the ACE gene roughly one-and-a-half times the odds of cough, and a variant in the bradykinin B2 receptor gene more than doubled the odds. The same genetic associations were not significant in people of European descent.⁶

A large genome-wide study using the UK Biobank and another cohort confirmed this pattern from a different angle. The overall prevalence of ACE inhibitor cough in those datasets was around 13 to 15 percent, but when broken down by genetic ancestry, the prevalence among individuals of East Asian descent was about 22 percent, compared to roughly 14 percent in Europeans. People of South Asian ancestry also showed a higher rate, around 16 percent.⁷ These ancestry-linked differences are not trivial; they can influence prescribing decisions, particularly in populations where the cough rate is high enough that a substantial number of patients may struggle to stay on the drug.

Beyond genetics, women appear to be more susceptible than men, and people who already have a sensitive cough reflex from other causes, such as asthma or chronic post-nasal drip, may be more likely to notice and be troubled by the cough. Smoking status also seems to matter, though the direction of the association is not always consistent across studies.

What Happens When the Cough Drives People Off Their Medication

ACE inhibitors are among the most effective drugs for managing high blood pressure, heart failure, and kidney disease related to diabetes. They reduce the risk of heart attacks, strokes, and death. So when a side effect pushes someone to stop taking one, the consequences can be real. About one in five patients ends up discontinuing an ACE inhibitor because of side effects, and the cough is the leading reason.⁸

Research on prescription patterns found that patients who developed a cough and were prescribed antitussive (cough-suppressing) medications had notably worse adherence to their ACE inhibitor. Only about 52 percent of those patients continued their therapy, compared to roughly 75 percent of patients who did not develop a cough.⁹ That gap in adherence is significant because uncontrolled blood pressure quietly accumulates cardiovascular damage over time. A cough that seems like a nuisance can lead to real health consequences if it causes someone to abandon a medication that was protecting their heart or kidneys.

Managing or Replacing the Medication

The most straightforward fix is switching to a different class of drug. Angiotensin receptor blockers, commonly called ARBs, target the same hormonal system but work at a different step. Instead of blocking the enzyme that degrades bradykinin, ARBs block the receptor for angiotensin II. Since they do not interfere with bradykinin breakdown, the cough rate with ARBs is essentially the same as with a placebo.¹⁰ Clinical trials comparing the two classes have consistently found lower cough incidence with ARBs, and the blood pressure-lowering effectiveness is comparable.¹¹ A systematic review and network meta-analysis concluded that ACE inhibitors should be avoided in patients at high risk of developing cough, with an ARB or calcium channel blocker offered as an alternative depending on the patient’s other health conditions.¹²

For patients who tolerate the ACE inhibitor well in every other respect and whose doctors prefer to keep them on it, a few smaller interventions have been explored. One small but intriguing trial tested iron supplementation and found a significant reduction in daily cough scores among patients who took iron compared to those on placebo.¹³ The proposed explanation involves iron’s ability to influence nitric oxide synthesis, which may blunt part of the inflammatory cascade downstream of bradykinin. That said, the study was small, and iron supplementation for this purpose has not become standard clinical practice. Cromolyn sodium, inhaled corticosteroids, and even nonsteroidal anti-inflammatory drugs like sulindac have all been tried in small studies with mixed results, but none has emerged as a reliably effective antidote that lets patients keep taking the ACE inhibitor without the cough.

How Bradykinin Cough Relates to Angioedema

Bradykinin does not just irritate nerve endings. At high concentrations it can also make blood vessels more permeable, causing fluid to leak into surrounding tissues. When this happens in the soft tissues of the face, lips, tongue, or throat, the result is angioedema, a potentially dangerous swelling that can obstruct the airway. ACE inhibitors can trigger angioedema in up to about 0.7 percent of patients, driven by the same underlying bradykinin accumulation that causes the cough. The two side effects share a common root but differ dramatically in severity: the cough is annoying and sometimes debilitating, while angioedema can be life-threatening.

Not everyone who develops the cough goes on to develop angioedema, and the two do not necessarily predict each other. Still, some clinicians view the cough as a signal that a patient may have an exaggerated bradykinin response, and use it as a reason to err on the side of switching medications rather than waiting to see whether the more serious reaction emerges.

Bradykinin Beyond Blood Pressure Drugs

Although ACE inhibitor cough is by far the most common clinical setting for bradykinin-driven airway irritation, the peptide itself is involved in a broader range of conditions. During the early years of the COVID-19 pandemic, researchers noticed that patients with severe disease had features consistent with a “bradykinin storm,” a hypothesis suggesting that viral infection dysregulated the kallikrein-kinin system, which produces bradykinin. A study of bronchoalveolar fluid from mechanically ventilated COVID-19 patients found elevated levels of bradykinin breakdown products and higher tissue kallikrein activity compared to patients with severe lung disease from other causes.¹⁴ The clinical relevance is still being sorted out, but the finding underscored that bradykinin is not just a medication side-effect curiosity; it is an active player in lung inflammation.

Hereditary angioedema, a rare genetic condition, is another context in which bradykinin overactivity is the central problem. People with this condition have a deficiency in the protein that normally keeps the kallikrein-kinin pathway in check, leading to episodic swelling attacks. Treatments for hereditary angioedema include drugs that directly block bradykinin B2 receptors, which brings the pharmacology full circle: the same receptor that drives ACE inhibitor cough is the target of a therapy designed to shut down bradykinin’s effects in a different disease.

Diagnosing the Cough When It Is Not Obvious

In practice, the diagnosis is clinical and retrospective. There is no blood test that measures “too much bradykinin in the airways” in a quick, routine way. Researchers have used capsaicin cough-sensitivity testing and skin responses to injected bradykinin and substance P to study how the cough reflex changes over time after ACE inhibitor withdrawal.¹⁵ These methods are useful in research settings but are not practical for everyday clinical use.

What typically happens is a process of elimination. A patient on an ACE inhibitor presents with a dry cough, and the doctor considers other common causes: post-nasal drip, asthma, gastroesophageal reflux, or respiratory infection. If none of those pans out, or if the timing lines up with starting the medication, the next step is usually to stop the ACE inhibitor for a few weeks. If the cough clears and then returns on rechallenge, the diagnosis is essentially confirmed. The challenge is that some patients have been on ACE inhibitors for years before they or their doctor considers the medication as the culprit, especially if the cough developed gradually.

For anyone dealing with a dry cough that has not responded to the usual remedies, checking whether you are on an ACE inhibitor is a worthwhile early step. Common ACE inhibitor names include lisinopril, enalapril, ramipril, perindopril, and captopril. If you recognize one on your medication list and the timeline fits, a conversation with your prescriber about a trial switch to an ARB can resolve the question relatively quickly and painlessly.