Severe acne refers to the presence of not only acne, papules, and pustules, but also multiple nodules and cysts. The first line of treatment for such patients is oral retinoids, along with the application of topical medications. The principles of topical application are the same as for inflammatory acne. Oral isotretinoin is the most effective treatment for this group of patients. For patients with more inflammatory papules and pustules, a combination of antibiotics and peroxymethylphenidate can be applied systematically before switching to oral isotretinoin when the lesions improve significantly. The dose of oral isotretinoin is calculated according to the weight of the patient. The common dose is 0.25~0.5 mg/kg/day. Since retinoic acid has more side effects, the oral dose should not exceed 0.5 mg/kg/day in order to reduce adverse reactions. The size of the cumulative dose is significantly associated with acne recurrence. The minimum cumulative dose is 60 mg/kg, and the dose can be gradually reduced to discontinuation after the acne has subsided and no new skin rashes have appeared. The duration of treatment depends on the lesion regression and the dose of the drug, and usually requires ≥16 weeks of continuous use. The main side effects of isotretinoin are dry lips and dry skin, which occur in 100% of patients. Isotretinoin will inhibit the secretion of sebaceous glands, resulting in dry skin mucosa, dry lips, and even bleeding nasal mucosa. This is a normal phenomenon. 2. Fetal teratogenicity. Women who intend to get pregnant should not take retinoic acid orally. Patients should be strictly contraceptive from 1 month before taking the drug to 3 months after stopping it. Patients who become pregnant accidentally during the medication must abort. 3. Epiphyseal deformity. It should not be taken by growing children. Retinoic acid can cause early epiphyseal closure in children, resulting in children not being able to grow taller. Therefore, children younger than 12 years old should not use it. In addition, the effects of isotretinoin on mental health, our acne treatment guidelines point out that isotretinoin causes depression or suicide and the correlation between drug use is unclear, because acne itself can also lead to low self-esteem and depression. It is recommended that it should not be used in patients who already have depressive symptoms or who are depressed. After severe acne has resolved, follow the principles of acne treatment (anti-inflammatory treatment followed by improvement of epithelial keratinization for acne) for follow-up treatment.