If you have painful veins or don’t like the way they look, you’re not alone. In fact, over 40 million people in the United States have varicose veins, and many of them may not know they have an underlying disease called Chronic Venous Insufficiency, according to United Vein Centers.

Chronic Venous Insufficiency happens when your leg veins don’t allow blood to flow back up to your heart because valves are damaged, weak, or defective.

“This can cause blood to collect (pool) in your legs,” United Vein Centers says. “Pooling of blood in the leg veins is the root cause of varicose and spider veins and their associated symptoms.”

Fortunately, you can get medical treatment to improve both the pain and appearance of vein conditions. Here’s how to identify the three most common vein conditions.

Spider veins

You’ll recognize the symptoms for this condition from the name, as spider veins are small and radiate out from one location, like spider legs.

They can appear on the surface of your legs or face and, while not harmful, the webbing of blue, red, or purple may bother you. Treatment can reduce the appearance of spider veins.

Varicose veins

Similar to spider veins, varicose veins develop from high blood pressure because of gravity, body weight, or hormonal changes.

“Varicose veins are bulging, twisted, and often discolored veins frequently found in the legs,” United Vein Centers says. “They are branches of other veins found deeper inside the leg.”

Varicose veins can cause leg pain, achiness, heaviness, throbbing, itching, burning, cramps, fatigue, restlessness, skin tightness, or swelling.

“If left untreated, varicose veins and their underlying cause can lead to serious problems like deep venous thrombosis (deep vein blood clots), thrombophlebitis (blood clot with vein inflammation), skin discoloration and thickening, and skin ulcers (open sores),” United Vein Centers says.

Deep vein thrombosis

The most serious of the three vein conditions is deep vein thrombosis, which occurs when a blood clot forms in your body, usually in your thigh or lower leg.

“The most serious complication of DVT happens when a part of the clot breaks off and travels through the bloodstream to the lungs, causing a blockage called pulmonary embolism” the Centers for Disease Control and Prevention says.

Deep vein thrombosis causes long-term swelling, pain, and ulcers in up to half of people who experience it. It can also be fatal if the clot is large enough to stop blood from reaching your lungs, according to the CDC.

Prevention and treatment

You can combat common causes of vein problems by losing weight, treating underlying illnesses or injuries, quitting smoking, wearing compression stockings, and increasing your activity level — so you spend less time sitting or standing in one spot.