Your body forms blood clots to stop bleeding when you’re injured. That’s a good thing. But sometimes a clot forms inside a vein or artery when it shouldn’t — and that can be dangerous. Knowing the warning signs of a harmful blood clot can make a life-saving difference.

What Makes a Blood Clot Dangerous?

Not all clots are harmful. When you cut yourself, clotting is your body’s natural way of healing. But when a clot forms inside a blood vessel or grows too large, it can block blood flow to vital organs.

Three of the most common and serious types of blood clots are:

• Deep vein thrombosis (DVT): A clot that forms in a deep vein, most often in the leg. DVT can cause pain and swelling. If part of the clot breaks loose, it can travel to the lungs.
• Pulmonary embolism (PE): A potentially life-threatening condition that happens when a DVT blood clot breaks off, travels to the lungs and blocks blood flow. A PE is a medical emergency.
• Arterial thrombosis (AT): A clot that forms in an artery. Because arteries carry oxygen-rich blood from the heart to vital organs, including the brain, these clots can cause a heart attack or stroke.

“One of the biggest challenges with blood clots is that the early symptoms — a little leg soreness, some mild swelling — can feel like nothing,” says Micah J. Eimer, MD, a cardiologist at Northwestern Medicine and a clinical assistant professor of Medicine at Northwestern University Feinberg School of Medicine. “But a clot that goes unrecognized can travel to the lungs and become a life-threatening emergency. When it comes to these symptoms, especially in someone with risk factors, I’d rather see a patient who turns out to be fine than miss the one who isn’t.”

Warning Signs of a Blood Clot

Deep vein thrombosis most often starts in the lower leg, thigh or pelvis, but it can also happen in the arm. Symptoms can be hard to notice, which is part of what makes DVT difficult to detect.

Watch for:

• Swelling in one leg or arm (usually just one side)
• Pain or tenderness, often described as a cramp or soreness
• Skin that feels warm to the touch
• Red or discolored skin over the affected area

Keep in mind that some people with DVT have no symptoms at all, which is why risk factors matter just as much as symptoms.

Warning Signs That a Clot Has Traveled to the Lungs

If a DVT breaks loose and travels to the lungs, it becomes a pulmonary embolism (PE). This is a medical emergency. Call 911 right away if you experience any of these symptoms:

• Sudden shortness of breath
• Sharp chest pain that may worsen with deep breathing
• A rapid or irregular heartbeat
• Coughing up blood
• Lightheadedness or fainting

PE symptoms can come on and worsen quickly. Do not wait to see if they improve on their own.

Warning Signs of a Stroke or Heart Attack From a Clot

When clots form in arteries, they can reduce or block blood flow to the brain or heart and trigger a stroke or heart attack. Time is critical. Know these signs:

Stroke symptoms (B.E. F.A.S.T.)

• Balance: Sudden loss of balance or coordination.
• Eyes: Sudden vision changes in one or both eyes.
• Face: One side of the face droops or feels numb.
• Arms: Arm weakness or numbness, especially on one side.
• Speech: Slurred speech, difficulty speaking or understanding speech.
• Time: Call 911 immediately if any of these symptoms occur and note when symptoms began.

Heart attack symptoms

• Chest discomfort, pressure or squeezing pain
• Shortness of breath
• Extreme fatigue or weakness
• Unexplained pain in your torso, back, neck, arms, shoulders or jaw
• Dizziness
• Cold sweat
• Nausea
• Upper abdominal discomfort
• Indigestion
• Anxiety

If you or someone near you has any of these symptoms, call 911 and seek medical attention right away.

“With a pulmonary embolism, stroke or heart attack, time is everything. The treatments we offer are very effective, but they have a window,” says Dr. Eimer. “Driving yourself to the emergency department or waiting to see if things improve can cost you that window.”

Risk Factors for Blood Clots

Certain factors raise the likelihood that a dangerous clot will form. You may be at higher risk if you:

• Recently had surgery, especially orthopaedic procedures like a hip or knee replacement
• Are pregnant or recently gave birth
• Have been immobile for a long period, such as during a long flight, car trip or hospital stay
• Have a personal or family history of blood clots
• Take hormonal medications, including certain contraceptives (birth control pills)
• Have cancer or are undergoing cancer treatment
• Are age 60 or older
• Have a condition such as heart disease, obesity or an inherited clotting disorder
• Smoke

Having one or more of these risk factors doesn’t mean you will develop a blood clot. It means you should pay close attention to any symptoms and talk to your doctor about prevention.

Everyday Habits That Can Raise Your Risk

Sitting for long periods of time is one of the most common contributors to blood clot risk in otherwise healthy people. When you sit for a long time — on a flight, during a long road trip or at a desk — blood can pool in your legs and blood clots can form more easily.

To lower your risk during periods of inactivity:

• Get up and walk around every hour or so.
• Flex and stretch your legs, ankles and feet while seated.
• Drink plenty of water to stay hydrated.
• Wear compression stockings during long flights or car trips, if your doctor recommends it.
• Do not cross your legs for long stretches.
• Maintain regular physical activity, which can help support healthy circulation.

Diagnosis and Treatment

If your doctor thinks you have a blood clot, they may recommend:

• Ultrasound: The most common test for DVT, using sound waves to detect clots in the veins
• CT scan or CT pulmonary angiography: Often used to diagnose pulmonary embolism
• Blood tests: Including a D-dimer test, which can indicate whether clotting is occurring in the body

Treatment depends on the type, size and location of the clot, as well as your overall health. Options may include:

• Blood thinners (anticoagulants): Medications that reduce the body’s ability to clot, helping prevent blood clots from growing and new clots from forming. These do not dissolve existing clots but give the body time to break them down naturally.
• Thrombolytics: Clot-dissolving medications used in more serious cases, such as pulmonary embolism.
• Procedures: In some cases, a filter may be placed in a large vein to prevent clots from traveling to the lungs, or a procedure may be used to remove the clot.

“Blood thinners often get a bad reputation, but for most patients, they are very well tolerated and highly effective,” says Dr. Eimer. “They don’t dissolve the clot overnight, but they stop it from growing and give your body the time it needs to do that work on its own. The most important thing is not to stop taking them without talking to your doctor first — that’s when we see problems.”

While not every blood clot can be prevented, understanding your risk factors and recognizing symptoms early can make a critical difference. A clot that reduces blood flow to the lungs, brain, heart or other parts of the body can quickly lead to serious health problems, including a pulmonary embolism, heart attack or stroke.

If you have a history of blood clots, are recovering from surgery, or expect to be inactive for long periods of time, talk with your doctor about steps you can take to lower your risk. Knowing what to watch for — and when to act — can help protect your health and potentially save your life.