How Does Aminocaproic Acid Work to Stop Bleeding?

Aminocaproic acid is a drug that slows bleeding by preventing blood clots from breaking down too quickly. Developed in the 1950s as one of the first synthetic antifibrinolytic agents, it has been used for decades in surgeries where major blood loss is expected, in brain hemorrhage emergencies, and in people with bleeding disorders like hemophilia. Though it has been somewhat eclipsed by a related drug called tranexamic acid, aminocaproic acid remains widely used, particularly in the United States, where its lower cost and long track record keep it relevant in operating rooms and intensive care units.

How It Prevents Bleeding

When you cut yourself, your body forms a clot to stop the bleeding. But your body also has a built-in system for dissolving clots once they’ve done their job, a process called fibrinolysis. In normal circumstances this balance works well. During major surgery or after certain injuries, though, the clot-dissolving machinery can become overactive, breaking down clots before wounds have properly healed. The result is bleeding that keeps going when it shouldn’t.

Aminocaproic acid works by blocking plasmin, the enzyme responsible for dissolving clots. It does this by binding to plasminogen, the inactive precursor of plasmin, and preventing it from attaching to fibrin, the protein mesh that holds a clot together. The net effect is that clots stay intact longer, giving surgical wounds and injured blood vessels more time to heal. Japanese researchers discovered the compound in the 1950s while systematically testing roughly 200 derivatives of the amino acid lysine for their ability to inhibit plasmin. That same research program also produced tranexamic acid, which works through the same mechanism but is several times more potent on a milligram-for-milligram basis.¹

Cardiac Surgery

Heart surgery on a bypass machine is one of the settings where aminocaproic acid has been studied most extensively. The bypass circuit exposes blood to artificial surfaces that activate clotting and fibrinolysis simultaneously, often leading to heavy bleeding once the operation is finished. A meta-analysis published in Circulation found that aminocaproic acid reduced total blood loss by about 35% compared with placebo, and cut the total number of postoperative transfusions by roughly 61%.² Those numbers were essentially identical to the results seen with high-dose aprotinin, a competing drug that was later pulled from the market over safety concerns. That withdrawal in 2007 pushed many cardiac surgery programs toward aminocaproic acid or tranexamic acid as their standard antifibrinolytic.

A more recent systematic review and meta-analysis comparing the two surviving drugs head-to-head in adults undergoing cardiopulmonary bypass found that tranexamic acid produced modestly lower 24-hour blood loss and fewer packed red blood cell transfusions. The safety profiles, however, looked similar: no significant difference in neurological events, kidney injury, cardiac complications, or blood clots between the two drugs.³ That review’s authors concluded that the evidence supports a preference for tranexamic acid in this setting, though they acknowledged the two drugs are close enough in performance that cost, availability, and institutional experience reasonably factor into the choice.

Spine Surgery

Major spine operations, especially those correcting scoliosis or other complex deformities, can involve hours of work on bone and muscle with extensive blood loss. Multiple randomized trials have examined aminocaproic acid in this context, typically using an intravenous loading dose followed by a continuous infusion throughout the procedure. A meta-analysis pooling these trials found that patients who received the drug had meaningfully lower postoperative and total blood loss and needed roughly 1.7 fewer units of transfused blood compared with patients who got a placebo.⁴

An interesting pattern emerged in at least one large trial of adolescents undergoing scoliosis correction: intraoperative blood loss was only marginally lower with the drug, but total blood loss during the entire hospital stay dropped substantially, from an average of about 4,100 mL per case down to roughly 2,400 mL. Researchers speculate this happened because the continuous infusion kept working long after the incision was closed, preventing the slow oozing that adds up over hours and days of recovery.⁵ Another randomized trial in adults undergoing major spinal reconstruction similarly found that while intraoperative blood loss did not differ significantly, postoperative transfusion needs were lower in the aminocaproic acid group.⁶

Brain Hemorrhage and Aneurysm Rebleeding

When a brain aneurysm ruptures, the initial bleed is dangerous enough, but rebleeding before the aneurysm can be surgically repaired or sealed with coils is often catastrophic. Aminocaproic acid has been used as a bridge therapy to stabilize the clot at the rupture site during the window between diagnosis and definitive treatment. One retrospective study at a large neurosurgery center found that rebleeding dropped from about 11% in untreated patients to under 3% in those who received an aminocaproic acid protocol.⁷

The picture is not uniformly positive, though. A different study looking specifically at short-term aminocaproic acid use in patients who went on to have endovascular coil embolization found no significant change in rebleeding risk.⁸ The discrepancy likely reflects differences in how quickly patients got to definitive treatment and how long the drug was given. A more recent trial found that combining aminocaproic acid with nimodipine, a drug that prevents spasm of brain arteries, reduced rebleeding more than nimodipine alone in the first week after hemorrhage, without increasing complications.⁹ Current practice has generally shifted toward ultra-early aneurysm repair, which shrinks the window where antifibrinolytic therapy matters. But in hospitals where delays to the operating room or angiography suite are unavoidable, the drug still has a role.

Eye Injuries

A traumatic hyphema, bleeding into the front chamber of the eye after a blow to the face, carries a risk of secondary hemorrhage that can raise eye pressure dangerously and threaten vision. Aminocaproic acid has been tested in this setting both as an oral medication and as a topical gel applied directly to the eye. In a controlled trial, only about 3% of patients receiving aminocaproic acid (either topically or by mouth) experienced a secondary hemorrhage, compared with 22% of untreated controls.¹⁰ An earlier placebo-controlled trial similarly showed that oral aminocaproic acid dramatically cut rebleeding rates.¹¹

A phase III trial of a topical gel formulation called Caprogel found rebleeding in 8% of the treatment group versus 30% of placebo patients, though the confidence interval for the difference between groups was wide enough that statistical significance was borderline.¹² Oral aminocaproic acid can cause nausea and dizziness in some patients, so the topical route is attractive for eye injuries if available, since it delivers the drug directly where it is needed with far fewer systemic side effects.

Pediatric Uses and ECMO

Children on extracorporeal membrane oxygenation, a form of life support that circulates blood through an external machine to take over for failing lungs or heart, face a tricky problem: the machine requires blood-thinning medication to prevent clots in the circuit, but that same blood-thinning raises the risk of devastating bleeding, especially intracranial hemorrhage in newborns. Aminocaproic acid has been used as a countermeasure. One early study in neonates and children on ECMO found that treated patients bled significantly less and needed fewer blood transfusions than untreated patients. Strikingly, no patient receiving the drug developed a new intracranial hemorrhage or had an existing one worsen.¹³

A more recent study of children on cardiac ECMO confirmed the bleeding-reduction effect, showing that average bleeding rates dropped from about 10.5 mL per kilogram per hour before starting the drug to under 4 mL per kilogram per hour within 24 to 48 hours, with corresponding drops in the need for red blood cell transfusions, plasma, platelets, and other blood products.¹⁴ In pediatric cardiac surgery without ECMO, a head-to-head comparison of aminocaproic acid and tranexamic acid found no significant difference in blood loss, reoperation for bleeding, seizures, kidney injury, or mortality between the two drugs.¹⁵ For children, where weight-based dosing and seizure risk are particular concerns, the interchangeability of the two drugs gives clinicians useful flexibility.

Bleeding Disorders and Dental Work

People with hemophilia face a specific challenge with dental procedures: the mouth is rich in enzymes that break down clots, making post-extraction bleeding hard to stop even with clotting factor replacement. Aminocaproic acid, given orally for several days around a dental procedure, helps stabilize clots in the mouth long enough for healing to occur. This approach has been described as an efficient and practical way to manage hemophilia patients who need dental work, sometimes reducing or eliminating the need for expensive clotting factor concentrates.¹⁶ Beyond hemophilia, aminocaproic acid has been used in patients with other bleeding disorders and in those with blood cancers who develop low platelet counts and mucosal bleeding.

Safety and Side Effects

The side effects most people experience with aminocaproic acid are relatively mild: nausea, diarrhea, dizziness, and nasal congestion. The more serious concern, at least in theory, is that a drug designed to keep clots from dissolving could tip the balance toward unwanted clotting, producing deep vein thrombosis, pulmonary embolism, or stroke. In practice, the evidence on this is reassuring. A study of patients with blood cancers who received the drug found thromboembolic complications in under 5% of cases, and all of those patients had multiple other risk factors for clots, including indwelling central catheters.¹⁷ A Bayesian network meta-analysis comparing various antifibrinolytic regimens in hip replacement surgery found no increased risk of pulmonary embolism or deep vein thrombosis with aminocaproic acid compared to placebo.¹⁸

The rarer but more alarming side effect is muscle damage. Case reports have documented a spectrum ranging from mild muscle pain to severe rhabdomyolysis, where muscle tissue breaks down rapidly and floods the kidneys with myoglobin, potentially causing kidney failure.¹⁹ Muscle biopsies in affected patients suggest the drug may have direct toxic effects on muscle fibers rather than causing damage through blood vessel obstruction.²⁰ This complication appears to be associated with prolonged use at high doses, particularly in patients receiving the drug for weeks, as was once common in the management of subarachnoid hemorrhage. Short-course perioperative use carries much lower risk. Patients on the drug for more than a few days are typically monitored for rising muscle enzymes in blood tests.

How the Body Handles the Drug

Aminocaproic acid is absorbed rapidly when taken by mouth and clears the body primarily through the kidneys. Studies in healthy volunteers found that about two-thirds of the drug is excreted unchanged in urine, with a half-life of roughly five hours.²¹ That heavy reliance on kidney clearance means patients with poor kidney function accumulate the drug much faster. Pharmacokinetic modeling in patients undergoing heart bypass surgery found that those with kidney failure had markedly reduced clearance, and authors recommended adjusted dosing regimens to maintain a stable blood concentration without overdosing.²² For patients on dialysis, the picture is even more striking: clearance drops to about a quarter of normal, meaning those patients should receive roughly a quarter of the standard dose.²³

Aminocaproic Acid Versus Tranexamic Acid

The comparison between these two drugs comes up constantly in surgical planning. Both block the same pathway, but tranexamic acid binds more tightly to its target and is effective at lower doses. In cardiac surgery, the accumulated evidence gives tranexamic acid a slight edge in reducing blood loss and transfusions, though the safety profiles are comparable.²⁴ In pediatric cardiac surgery, the drugs appear interchangeable.²⁵ And in joint replacement, topical aminocaproic acid has been found non-inferior to tranexamic acid for reducing blood loss while costing considerably less.²⁶

Cost is where aminocaproic acid often wins the argument. In many hospital formularies, it is substantially cheaper than tranexamic acid, sometimes by an order of magnitude. A study conducted during a national medication shortage, when one of the drugs temporarily disappeared from the supply chain, found that switching between them produced equivalent postoperative bleeding and transfusion rates, suggesting the two are functionally interchangeable for most cardiac surgery patients.²⁷ In resource-limited settings or institutions where budgets are tight, aminocaproic acid’s lower price can be the deciding factor. The clinical difference between the two drugs is small enough that economics and availability reasonably tip the scale.

Veterinary Uses

Aminocaproic acid has found a niche in veterinary medicine as well. Retired racing greyhounds, a breed known for unusual bleeding tendencies after surgery, showed a significant reduction in delayed postoperative bleeding when given the drug. In a placebo-controlled trial, 30% of untreated greyhounds bled after routine spay or neuter surgery, compared with 10% of those who received aminocaproic acid. The drug appeared to work by increasing clot strength.²⁸ The drug has also been used in emergency veterinary settings: a case report described rapid clinical improvement and diminished clot breakdown in a dog with suspected acute traumatic coagulopathy after treatment with aminocaproic acid.²⁹ Greyhound rescue organizations have become especially aware of this drug, and many veterinarians who work with the breed now consider it a standard part of the perioperative toolkit.