During menstruation, it is not absolutely impossible to dye your hair, but you should try to avoid doing it during the special physiological period. If the skin is usually sensitive, it is not recommended to dye hair during menstruation, mainly due to the cyclical exfoliation and bleeding process of the endometrium that occurs during menstruation, which temporarily reduces the body’s resistance and increases skin sensitivity. Most of the hair dyes on the market contain various toxic and harmful chemicals, such as lead, mercury, p-phenylenediamine, oxidants and so on, which will not only damage the hair surface scales, hair follicles and so on, making the hair yellow, dry, fragile and easy to break and fall off. It may also be absorbed into the body’s circulation through the rich network of blood vessels in the damaged scalp, resulting in local or systemic skin allergies, eczema and dermatitis and other uncomfortable symptoms, long-term frequent use may also increase the risk of cancer.