There are two parts to thrombophlebitis, inflammation of a vein and blood clot formation. If the inflammation is minor, the disease usually is called venous thrombosis or phlebothrombosis. Thrombophlebitis can occur in both deep veins and superficial veins, but most often occurs in the superficial veins of the extremities (legs and arms). Most cases occur in the legs. When thrombophlebitis occurs in a superficial vein, one that is near the surface of the skin and is visible to the eye, the disease is called superficial thrombophlebitis. Any form of injury to a blood vessel can result in thrombophlebitis. In the case of superficial thrombophlebitis, the blood clot usually attaches firmly to the wall of the affected blood vein. Since superficial blood veins do not have muscles that massage the veins, blood clots in superficial veins tend to remain where they form and seldom break loose. When thrombophlebitis occurs in a deep vein, a vein that runs deep within muscle tissue, it is called deep venous thrombosis. Deep venous thrombosis presents the threat of producing blood clots that will break loose to form emboli. These can lodge in other tissues where they can block the blood supply, typically in the lungs. This results in tissue damage and can sometimes be serious or fatal, for example; pulmonary embolism.

The main symptoms are tenderness and pain in the area of the affected vein. Redness and/or swelling also may be seen. In the case of deep venous thrombosis, there is more swelling than is caused by superficial thrombophlebitis, and the patient may experience muscle stiffness in the affected area. There are many causes of thrombophlebitis.

The main causes can be grouped into three categories; injury to blood veins, increased blood clotting, and blood stasis. When blood veins are damaged, collagen in the blood vein wall is exposed. Platelets respond to collagen by initiating the clotting process. Damage to a vein can occur as a consequence of indwelling catheters, trauma, infection, Buerger's disease, or the injection of irritating substances. Increased tendency of the blood to clot can be caused by malignant tumors, genetic disorders, and oral contraceptives, though newer generation birth control pills carry a lower risk for many women. Stasis, in which the blood clots due to decreased blood flow in an area, can happen following surgery, as a consequence of varicose veins, as a complication of postpartum states, and following prolonged bed rest. In the case of prolonged bed rest, blood clots form because of inactivity, which allows blood to move sluggishly and stagnate (collect) in blood veins. This can lead to blood clots. These clots (also called emboli) sometimes are released when the patient stands up and resumes activity. This can present a problem if the emboli lodge in vital organs. In the case of postpartum patients, a fever developing four to 10 days after delivery may indicate thrombophlebitis. A 2004 study revealed that postmenopausal women taking hormone therapy combining estrogen and progestin had more than a twofold higher risk of venous thrombosis than non-hormone users.

In superficial thrombophlebitis, the location of the clot sometimes can be seen by the unaided eye. Blood clots are hard and can usually be detected by a physician using palpation (touching or massage). Deep venous thrombosis requires specialized diagnostic procedures to detect the blood clot. Among the exams a physician may use are ultrasound and x ray, coupled with dye injection (venogram).

Superficial thrombophlebitis usually resolves without treatment. If treatment of superficial thrombophlebitis is given, it usually is limited to the application of heat or anti-inflammatory drugs, like aspirin or ibuprofen, which also help to relieve the pain. It can take from several days to several weeks for the clot to resolve and the symptoms to completely disappear. A 2003 study showed that low intensity therapy with warfarin, a common blood thinner, prevented recurrent venous thromboembolism in study subjects. A newer anticoagulant called ximelagatran also has been shown as equally or more effective than warfarin in preventing deep vein thrombosis.

Deep venous thrombosis is a serious condition, treated with anticoagulant drugs and by keeping the affected limb elevated. The primary objective in treating deep venous thrombosis is prevention of a pulmonary embolism. The patient usually is hospitalized during initial treatment. The prescribed anticoagulant drugs limit the ability of blood clots to grow and new clots to form. Sometimes, a drug that dissolves blood clots is administered. These drugs must be used with caution because, as the clot dissolves, it may release from the site where it formed and become an embolus. Surgery may be used if the affected vein is likely to present a long–term threat of producing blood clots that will release emboli. When superficial thrombophlebitis occurs in the groin, where the superficial veins join the deep veins, the threat of emboli is present. In this case, blood clots formed in the superficial veins can extend into the much larger deep vein where they break off and are released into the blood stream. The affected veins are either removed or tied off to prevent the release of the blood clots. Tying off superficial blood veins is an outpatient procedure that can be performed with local anesthesia. The patient is capable of immediately resuming normal activities.

Superficial thrombophlebitis seldom progresses to a serious medical complication, although non-lethal embolisms may be produced. Deep venous thrombosis may lead to embolism, especially pulmonary embolism. This is a serious consequence of deep venous thrombosis, and sometimes is fatal.