If patients have darkened lips, there are usually several reasons: First, the most common factor is cardiogenic factors, such as patients with atherosclerotic heart disease, unstable angina, myocarditis or pericarditis, endocarditis and other transient myocardial ischemia, resulting in insufficient blood supply and oxygen supply to the heart, which can lead to darkening of the lips for the above reasons. Second, related to chronic pulmonary insufficiency, for example, patients with pulmonary hypertension or chronic pulmonary heart disease, pulse astringency, chronic obstructive emphysema, chronic bronchial dilatation, when the arterial oxygen saturation is reduced, the partial pressure of carbon dioxide is too high and the partial pressure of oxygen is reduced, it will cause the lips of the mouth to turn black due to insufficient oxygen saturation. Thirdly, when poisoning diseases such as nitrite poisoning or acute harmful gas poisoning, there will be patients with blackened lips and mouth.