Blood clots can save your life when they form to stop bleeding after an injury. But when clots develop where they shouldn’t—or fail to dissolve on their own—they can turn dangerous fast. In fact, untreated blood clots can lead to life-threatening conditions like deep vein thrombosis (DVT), pulmonary embolism (PE), heart attack, or stroke.

The scary part? Many people don’t realize the early warning signs until it’s too late. That’s why learning how to recognize the red flags can make all the difference.

Below are 8 warning signs of blood clots you should never ignore.

1. Swelling in One Leg or Arm

If one leg suddenly swells up—especially around the calf—it could be a sign of a clot blocking blood flow. Unlike normal swelling from standing too long, clot-related swelling is usually one-sided and may feel painful or heavy.

2. Unexplained Pain or Tenderness

Sharp, throbbing, or cramping pain in your leg or arm without an obvious injury could mean trouble. The pain might worsen when you walk, stand, or flex your foot upward.

3. Skin That Feels Warm to the Touch

Clots block circulation, causing heat to build up in the affected area. If your leg or arm feels noticeably warmer than the surrounding skin, take it as a serious warning.