Call 911 now if you have any of these. A blood clot can become life-threatening within minutes.
- Pulmonary embolism (clot in the lungs): sudden shortness of breath, chest pain that is sharp or worsens when you breathe, coughing up blood, a racing heartbeat, lightheadedness, or fainting.
- Stroke (clot in the brain) — think FAST: Face drooping, Arm weakness, Speech difficulty, Time to call 911. Also sudden numbness on one side, sudden vision loss, severe headache, or trouble walking.
- Heart attack: chest pressure or pain, pain spreading to the arm, jaw, or back, shortness of breath, cold sweat, or nausea.
- Acute limb ischemia (clot blocking an artery in a limb): a suddenly cold, pale, painful, or numb arm or leg.
Do not drive yourself. Do not wait to see whether symptoms pass. If you take a blood thinner, do not stop it on your own — but still seek emergency care for the warning signs above.
Every year, blood clots — specifically venous thromboembolism (VTE) — affect up to 900,000 Americans and kill an estimated 60,000 to 100,000, making this condition a leading cause of preventable hospital death, according to the CDC. Yet public awareness remains low; surveys have repeatedly found that most people cannot identify the major signs of a blood clot. These are not obscure medical events — they can happen to anyone, at any age, and recognizing the symptoms quickly can be the difference between a treatable condition and a fatal one. For context on how vascular conditions connect to overall health, visit our medical conditions guide.
Quick summary. There are two broad danger categories. Venous clots — deep vein thrombosis (DVT) in a leg and pulmonary embolism (PE) in the lungs — are the focus of this guide. Arterial clots cause strokes, heart attacks, and blocked limbs. One-sided leg swelling with pain or warmth needs same-day evaluation; sudden breathlessness, chest pain with breathing, coughing blood, stroke signs, or a cold painful limb are 911 emergencies. Treatment centers on anticoagulants (“blood thinners”), which must be taken exactly as prescribed and never stopped without medical guidance. This article is general education, not medical advice.
What Are Blood Clots?
Blood clotting (coagulation) is a normal, life-saving process. When you cut yourself, platelets and clotting proteins rapidly form a clot to stop bleeding. Problems arise when clots form inside blood vessels where they aren’t needed, or when they fail to dissolve after serving their purpose.
The two major types of pathologic blood clots are arterial clots and venous clots. Arterial clots typically form in arteries narrowed by atherosclerosis and are the cause of most heart attacks and strokes; because they cut off oxygen to vital tissue, they are almost always sudden emergencies. Venous clots — the main focus of this guide — form in veins, usually the deep veins of the legs or pelvis. According to the CDC, venous thromboembolism encompasses two related conditions: deep vein thrombosis (DVT), a clot in a deep vein, and pulmonary embolism (PE), which occurs when a DVT fragment breaks free and travels to the lungs.
Superficial thrombophlebitis — clotting in a vein near the skin’s surface — is generally less dangerous than DVT, causing localized pain, redness, and swelling along the affected vein. However, superficial clots near the junction with the deep venous system can extend and become a DVT, so they shouldn’t be entirely dismissed. Any new leg symptom that is worsening deserves medical attention.
Causes and Risk Factors
In 1856, German pathologist Rudolf Virchow identified three broad categories of factors that predispose to clot formation — collectively known as Virchow’s triad: stasis (slowed blood flow), endothelial injury (damage to the blood vessel wall), and hypercoagulability (increased tendency of blood to clot). Most blood clots result from a combination of these factors.
Common situational risk factors include surgery (especially orthopedic procedures — hip and knee replacement carry among the highest risks), hospitalization and immobility, long-distance travel (flights or drives lasting more than four hours), pregnancy and the postpartum period, and the use of estrogen-containing medications (oral contraceptives and hormone replacement therapy). According to the NHLBI, surgery and hospitalization account for a large share of all VTE events, which is why clot prevention is a routine part of inpatient care.
Medical risk factors include cancer (which increases VTE risk several-fold), prior VTE (recurrence is common without ongoing anticoagulation for unprovoked events), obesity, heart failure, inflammatory bowel disease, nephrotic syndrome, and infection — including COVID-19, which is associated with a prothrombotic state. Age is a major factor — VTE risk approximately doubles with each decade after age 40.
Inherited thrombophilias — genetic conditions that increase clotting tendency — include Factor V Leiden mutation (present in about 5 percent of people of European descent), prothrombin G20210A mutation, protein C deficiency, protein S deficiency, and antithrombin deficiency. Acquired thrombophilia, particularly antiphospholipid syndrome, also significantly increases VTE risk. If you have a family history of clots at a young age, ask your doctor whether testing is appropriate.
Symptoms of Deep Vein Thrombosis (DVT)
DVT most commonly occurs in the legs, though it can develop in the arms (upper extremity DVT, often associated with central venous catheters or thoracic outlet syndrome) or other veins. According to the Mayo Clinic, symptoms of leg DVT include swelling in one leg (asymmetric leg swelling is a key finding), pain or tenderness in the leg (often starting in the calf and described as a cramp or soreness), redness or discoloration of the skin, and warmth over the affected area. Symptoms usually affect just one leg.
Importantly, roughly half of DVTs produce no noticeable symptoms, making them difficult to detect without imaging. When symptoms are present, they typically affect only one leg. Bilateral symptoms are less common and may suggest an alternative diagnosis or a large clot blocking the iliac veins or inferior vena cava. Because an untreated DVT can send a clot to the lungs, one-sided leg swelling with pain, warmth, or redness warrants same-day medical evaluation — do not wait to see if it improves overnight.
Post-thrombotic syndrome (PTS) is a long-term complication of DVT that develops in 20 to 50 percent of patients. It results from damage to the venous valves caused by the clot, leading to chronic venous insufficiency. Symptoms include persistent leg swelling, pain, heaviness, skin discoloration (particularly brown pigmentation around the ankles), and, in severe cases, venous ulcers. Prompt treatment and, in some patients, compression stockings can reduce this risk.
Symptoms of Pulmonary Embolism (PE)
Pulmonary embolism occurs when a clot — usually originating from a leg DVT — travels through the bloodstream and lodges in a pulmonary artery, blocking blood flow to a portion of the lung. PE is a medical emergency. The CDC estimates that sudden death is the first symptom in about one in four people with PE, which is why the warning signs must never be brushed off.
Symptoms of PE include sudden shortness of breath (the most common symptom), sharp chest pain that worsens with deep breathing or coughing (pleuritic chest pain), rapid heart rate (tachycardia), cough (sometimes with blood-tinged sputum), lightheadedness or fainting, and anxiety or a sense of impending doom. A massive PE can cause cardiovascular collapse, severe low blood pressure, and loss of consciousness.
PE is a 911 emergency. Call 911 immediately if you have sudden shortness of breath, chest pain that worsens with breathing, coughing up blood, a rapid heartbeat with lightheadedness or fainting, or leg swelling accompanied by any of these respiratory symptoms. Pulmonary embolism is dangerous but highly treatable when caught early — every minute counts.
Diagnosis
Diagnosis of DVT begins with clinical assessment using validated scoring systems — the Wells score assigns points based on clinical features to stratify patients into low, moderate, and high probability categories. D-dimer, a blood test that measures a fibrin degradation product, is useful as a “rule-out” test: a negative D-dimer in a low-probability patient effectively excludes DVT. However, D-dimer is elevated in many conditions (infection, inflammation, pregnancy, cancer, recent surgery) and is therefore not specific for clots.
Compression ultrasonography is the primary imaging test for DVT. It is non-invasive, widely available, and highly accurate for proximal DVT (above the knee), with sensitivity and specificity both above 95 percent. For suspected calf-vein DVT or when ultrasound is inconclusive, repeat ultrasound, CT venography, or MR venography may be used.
For suspected PE, CT pulmonary angiography (CTPA) is the gold-standard imaging test, with sensitivity exceeding 95 percent. According to the Radiological Society of North America, CTPA provides rapid, detailed visualization of the pulmonary arteries and can also reveal alternative diagnoses. Ventilation-perfusion (V/Q) scanning is an alternative for patients who cannot receive CT contrast (for example, severe contrast allergy or kidney impairment). Echocardiography can detect right ventricular strain from a large PE and help gauge the urgency of treatment.
Treatment
Anticoagulation: The Foundation of Treatment
Anticoagulation (“blood thinners”) is the mainstay of VTE treatment. These medications don’t dissolve existing clots but prevent clot extension and new clot formation, allowing the body’s natural fibrinolytic system to gradually break down the thrombus. According to the American Society of Hematology, direct oral anticoagulants (DOACs) — including rivaroxaban (Xarelto), apixaban (Eliquis), edoxaban (Savaysa), and dabigatran (Pradaxa) — are now preferred over warfarin (Coumadin) for most patients with VTE. The specific drug, dose, and duration are individualized by your clinician; do not adjust or stop these medications on your own, as doing so can allow a new clot to form.
DOACs offer several advantages: predictable dosing without routine blood monitoring, fewer drug and food interactions, rapid onset of action, and at least equivalent efficacy with potentially lower bleeding risk. Some DOACs can be used as single-drug therapy from the start, while others require a few days of initial treatment with an injectable anticoagulant (typically low-molecular-weight heparin). Your prescriber will choose the approach that fits your kidney function, other medications, and overall risk.
Warfarin remains appropriate in certain situations, including patients with mechanical heart valves, antiphospholipid syndrome, severe kidney impairment, or those for whom DOAC cost is a barrier. When warfarin is used, it is overlapped with heparin until blood tests (the INR) reach the target range. Whatever the drug, the most important safety rule is consistency: take it exactly as directed, keep follow-up appointments, and tell every provider (including dentists) that you are anticoagulated.
Duration of Anticoagulation
Treatment duration depends on whether the VTE was provoked (triggered by a temporary risk factor) or unprovoked. For VTE provoked by a major transient risk factor (surgery, major trauma), about three months of anticoagulation is standard. For unprovoked VTE or VTE associated with a persistent risk factor (such as cancer or ongoing immobility), extended or indefinite anticoagulation is typically recommended, because the recurrence risk without ongoing treatment is substantial. The decision to continue or stop is reassessed periodically and balances clot risk against bleeding risk — a conversation to have with your clinician rather than a decision to make alone.
Advanced Treatments
For massive PE with hemodynamic instability (low blood pressure, shock), systemic thrombolysis (clot-dissolving medication) can be lifesaving. Catheter-directed therapy — delivering thrombolytic drugs directly to the clot or mechanically removing it — is increasingly used for selected higher-risk PE with right ventricular strain but stable blood pressure. Surgical embolectomy is reserved for the most critical cases when thrombolysis is contraindicated or has failed.
Inferior vena cava (IVC) filters — small devices placed in the IVC to catch traveling clots — are used in patients with VTE who cannot take anticoagulation (for example, active bleeding or upcoming surgery). Retrievable IVC filters should be removed once the contraindication to anticoagulation resolves, as leaving them in place long-term carries risks of IVC thrombosis and filter migration.
Prevention
VTE prevention (thromboprophylaxis) is one of the most important patient-safety measures for hospitalized patients. The American College of Chest Physicians recommends pharmacologic prophylaxis (low-molecular-weight heparin or low-dose unfractionated heparin) for most hospitalized medical and surgical patients at increased VTE risk. Mechanical prophylaxis with intermittent pneumatic compression devices is used when medication-based prophylaxis is contraindicated.
After orthopedic surgery — particularly total hip and knee replacement — extended prophylaxis for several weeks is recommended because of the prolonged high-risk period. For cancer patients undergoing surgery, extended prophylaxis is also standard, and selected high-risk outpatients receiving chemotherapy may benefit from preventive anticoagulation. Your surgical or oncology team will tailor the plan.
For healthy individuals, common-sense prevention during high-risk situations includes staying active during long flights or car trips (walk the aisle, do calf exercises, stay hydrated), discussing VTE risk with your doctor before surgery, maintaining a healthy weight, staying physically active, and understanding that estrogen-containing medications increase risk. Knowing your family history of blood clots can also inform risk-reduction strategies. Understanding the cost of prevention versus treatment underscores why proactive measures are worthwhile.
Frequently Asked Questions
Can you feel a blood clot forming?
Not always. About half of DVTs produce no symptoms. When symptoms do occur, they typically develop gradually over hours to days — a progressive ache, swelling, or warmth in one leg. You won’t necessarily feel the clot “forming” as a discrete event. What you may notice is a change: one leg becomes more swollen, painful, or warm than the other. Any unexplained one-sided leg swelling, especially with pain, should prompt medical evaluation the same day.
How serious is a blood clot in the leg?
A DVT is a serious condition that requires treatment. The immediate danger is that a piece of the clot can break off and travel to the lungs, causing a pulmonary embolism — which can be fatal. With prompt anticoagulation, the risk of PE is greatly reduced, and most patients recover fully. The long-term concern is post-thrombotic syndrome, which develops in a substantial minority of DVT patients and causes chronic leg symptoms. Early treatment and, in some cases, compression stockings can reduce this risk.
Can blood clots go away on their own?
Small superficial blood clots may resolve without treatment. However, DVTs and PEs should never be left untreated. While the body’s natural fibrinolytic system does work to dissolve clots over time, anticoagulation is essential to prevent clot extension, PE, and recurrence while dissolution occurs. Without treatment, the risk of fatal PE and long-term complications is unacceptably high. Most appropriately treated DVTs show significant resolution over weeks to months.
Does aspirin prevent blood clots?
Aspirin is an antiplatelet agent that primarily helps prevent arterial clots (the type involved in heart attacks and strokes). It has limited effectiveness for venous clots (DVT/PE), which are driven more by the coagulation cascade than by platelet aggregation. Some studies suggest a modest benefit of aspirin for VTE prevention after orthopedic surgery or as extended prevention after completing initial anticoagulation. However, aspirin is not a substitute for proper anticoagulation to treat an existing venous clot, and you should not start or stop aspirin for this purpose without medical advice.
Can you fly with a blood clot?
If you have an acute DVT or PE, air travel should generally be avoided until you have been on stable anticoagulation for an adequate period — many clinicians advise waiting at least two to four weeks. Long flights can worsen venous stasis and increase the risk of clot extension. Once adequately anticoagulated, most patients with stable VTE can travel safely with precautions: wear compression stockings, stay hydrated, walk the aisle periodically, and take medication as prescribed. Always consult your doctor before flying with a recent or current blood clot.
What should I do if I miss a dose of my blood thinner?
Follow the specific instructions your pharmacist or prescriber gave you, since the guidance differs by medication. As a general rule, do not double up to “catch up,” and never stop the medication on your own. If you are unsure, call your pharmacy or prescriber. Sudden interruption of anticoagulation can allow a new clot to form, so consistency is essential.
Recognizing the Signs Could Save Your Life
Blood clots are common, dangerous, and treatable — but the treatment window matters. If you develop sudden swelling in one leg, particularly with pain or warmth, see a doctor the same day. If you experience sudden shortness of breath, chest pain with breathing, coughing up blood, a rapid heartbeat with fainting, or stroke warning signs — call 911. These are not symptoms to monitor overnight or manage with home remedies.
Know your risk factors. If you’re having surgery, ask your surgeon about VTE prevention. If you’re taking estrogen-containing birth control or hormone replacement, understand that this modestly increases your VTE risk. If you have a family history of blood clots, particularly at a young age, discuss thrombophilia testing with your doctor. If you’ve had a VTE before, understand your recurrence risk and discuss the appropriate duration of anticoagulation.
The statistics are clear: tens of thousands of VTE deaths occur annually in the United States, and the majority are preventable with awareness, prevention, and prompt treatment. Knowledge of the symptoms and risk factors is the single most important tool you have — and it costs nothing. This guide is educational and does not replace personalized advice from your own clinicians.