Dark red round patches on the back are painless and itchy, consider florid chafing rash, seborrheic dermatitis and pityriasis rosea, and choose the appropriate treatment according to the different causes of the disease. 1. florid chafing rash: florid chafing rash is a fungal infection of the skin caused by Malassezia violating the keratin of the skin, mainly the use of antifungal drug treatment, you can apply econazole, miconazole, ketoconazole, but also selenium disulfide lotion, benzoyl peroxide, and if the effect of the local drug treatment is not satisfactory, you can also take itraconazole, fluconazole orally. 2. Seborrheic dermatitis: Seborrheic dermatitis is a chronic inflammation that occurs in the seborrheic area, and can be treated with calcium-modulated phosphatase inhibitors, such as tacrolimus ointment, pimecrolimus cream, and selenium sulfide lotion or salicylic acid preparations. 3. pityriasis rosea: pityriasis rosea is a self-limiting inflammatory skin disease, the purpose of treatment is to reduce the symptoms and shorten the course of the disease, you can use glycerine lotion or apply glucocorticoid ointment, such as hydrocortisone cream, betamethasone cream and so on. All of the above medications should be used under the guidance of a doctor, avoid self-medication to avoid adverse consequences.