Red pimples on the back and forehead can be caused by acne, folliculitis and contact dermatitis. 1. Acne: Acne can be caused by increased androgen levels, bacterial infections and other factors, and is commonly found on the face, chest, back and other places, usually with pimples, papules and pustules and other symptoms. 2. Folliculitis: The formation of folliculitis is usually associated with Staphylococcus aureus infections, and can also be localized by excessive sweating and hair plucking. The disease may appear in any area with hair, commonly red papules, papules and pustules, etc. Some patients may have red pimples on the back and chest. 3. Contact dermatitis: contact dermatitis can be caused by exogenous substances such as chemical, animal and plant infections of the skin, the patient’s skin will appear erythema, edema and papules and other symptoms. There are many reasons for red pimples on the back and chest, so it is recommended that the patient consult a doctor for examination to clarify the cause of the disease.