Isotretinoin is an isomer of all-trans retinoic acid. Pharmacological activities: 1. inhibits keratin-forming cell growth. 2. inhibits sebaceous gland differentiation and sebum production. 3. anti-inflammatory activity, inhibits chemotaxis of neutrophils and release of lysosomes. 4. inhibits collagen synthesis in skin tissues. 5. low dose, immunostimulatory effect; high dose, immunosuppressive effect. It is mainly used clinically for the treatment of acne, severe facial seborrhea, psoriasis, and certain keratotic skin diseases. The usage is oral, the starting dose is 0.5mg/kg per day, and the dose is adjusted after 4 weeks, and can be increased to 1.0mg/kg per day for well-tolerated people; 0.1mg/kg per day for poorly tolerated people, and 16 weeks is a course of treatment, and the drug effect can still last for a period of time after stopping, and the drug must be stopped for 8 weeks before proceeding to the next course of treatment.