The most common cause of heart murmurs in adults is any condition that affects the heart valves. Heart murmurs can also be caused by hyperthyroidism, pregnancy, strenuous physical activity, fever, and anemia. Any alteration in the structure of the heart, including heart surgery, infection, or aging, can also be a trigger. Some of the specific illnesses that cause abnormal heart murmurs in adults include mitral valve prolapse, valve calcification, endocartitis and rheumatic fever. There are two kinds of heart murmurs, innocent and abnormal.

Abnormal heart murmurs are the most common variety experienced by adults, though it is possible to have an innocent heart murmur at any age. Innocent heart murmurs in adults can arise from any condition that provides an increased flow of blood to the heart, such as pregnancy or anemia. One of the most common ways an adult can get an innocent heart murmur is from mitral valve prolapse. This minor heart condition can be detected at birth or happen later in life. It contributes to heart murmurs because of a bulging mitral valve that slightly retreats back into the left atrium and allowing a tiny amount of blood to leak back into the heart chamber.

Most abnormal heart murmurs in adults arise due to heart valve disease, which is usually caused by another condition. Heart murmurs can also be caused by any incident that puts strain on the heart valves. They can develop due to the aftermath of a heart attack, high blood pressure, heart failure, or as the result of fatty build-up in the arteries.

Changes in the heart due to aging are another common cause of abnormal heart murmurs in adults. Valve calcification, a process by which substances such as calcium are deposited on the heart valves, is a typical condition of aging. These deposits cause a strain on the cardiovascular system and thus change the flow of blood, resulting in a heart murmur.

Conditions outside of the heart may also contribute to abnormal heart murmurs in adults. Germs in the blood can cause infections, such as endocartitis, which reach the heart and lead to murmurs. Rheumatic fever causes scarring on the heart valves which may not affect the flow of blood until years after recovery from the illness. Hyperthyroidism or high blood pressure are also causes of heart murmurs. Strenuous physical activity can also increase the flow of blood to such a degree that it triggers the condition.