Common causes of purple lips include the following.
Food poisoning, where nitrite poisoning can cause purple lips.
Some common respiratory diseases such as chronic bronchitis, bronchiectasis, emphysema, pulmonary heart disease and pulmonary hypertension can cause a decrease in oxygen saturation, resulting in purple lips.
Some cardiogenic diseases such as precardiac disease and pericarditis with heart valve disease can cause tissue hypoxia and result in purple lips.