Unstable angina is a pain or discomfort in the chest or areas around it. It is caused by inadequate blood flow to the heart. This should be distinguished from stable angina, which is a heart attack, and non-cardiac chest pain. All these can be differentiated on the basis of their pattern, severity, and frequency of symptoms and the also level of activity that precipitates the discomfort.

The most common cause of unstable angina is Coronary artery disease. In less than 10% of cases, a coronary artery spasm may cause angina. Narrowing (plaques) of the coronary arteries occurs and may be severe enough to cause stable angina with exertion or stress, or may not be severe enough to show any symptoms at all. For reasons not understood, at some point these plaques may rupture and platelets and blood clots may form, causing a relatively stable narrowing to become unstable or "high-grade"), limiting bloodflow to a region of the heart even at rest. In unstable angina, chest pain may occur at rest, or there may be increase in the severity, frequency, or duration of the pain, with chest pain occurring at lower levels of activity. There may be a history of MI (heart attack). Diabetes mellitus, hypertension, myxedema, peripheral vascular disease, heart valve disease, cardiomyopathy , and atherosclerosis may also produce angina.

Men are prone to this disease. Cigarette smoking, high cholesterol levels (in particular, high LDL cholesterol andlow HDL cholesterol) , high blood pressure, diabetes, a family history of coronary heart disease before the age of 55, sedentary lifestyle, and being more than 30% over ideal body weight can also cause this condition. Occasionally, sudden overwhelming stress can precipitate an episode ofangina.

The best prevention is to change the causal factors, that can be modified like stop smoking, losing weight if overweight, and control on blood pressure, diabetes and cholesterol. Some studies have shown that modifying these factors can prevent the progression ofarterial blockages and leading to decrease in the severity of blockages. Aspirin, antianginal medications such as nitrates (nitroglycerin), beta-blockers, calcium channel blockers, or others may be prescribed to prevent the occurrence of angina and lessen its severity.