The difference between Penciclovir and Acyclovir is mainly due to different dosage forms, different pharmacological and toxicological effects, and different adverse reactions. Penciclovir and acyclovir belong to nucleoside antiviral drugs, mainly for the treatment of viral infectious diseases. 1. Different dosage forms: Penciclovir often includes Penciclovir cream and Penciclovir injection; Acyclovir has more dosage forms in the clinic, and the common ones are Acyclovir tablets, Acyclovir gel, Acyclovir drops, Acyclovir cream, and so on. 2. Pharmacological and toxicological effects are different: Penciclovir triphosphate and deoxyguanine nucleoside triphosphate competitive inhibition of herpes simplex virus polymerase, thereby selectively inhibiting the synthesis and replication of herpes simplex virus DNA; Acyclovir interferes with viral DNA polymerase and inhibits viral replication, herpes simplex virus, varicella zoster virus, cytomegalovirus, and other inhibitory effects. 3. Adverse reactions are different: after the use of Penciclovir, there is no systemic adverse reactions, occasionally see the use of local burning sensation, pain, itching and other discomforts; Acyclovir external use, can be seen as mild pain, burning, tingling, itching, and skin rashes and so on. It is recommended that patients use the medication strictly in accordance with the doctor’s instructions, not blindly self-medication, to avoid adverse reactions, delay the condition.