The feeling of pain when injecting penicillin is mainly related to the reasons that penicillin is more irritating, penicillin particles are too large and not easy to be absorbed. 1. Penicillin is more irritating: penicillin needs to be injected into the muscle, and penicillin contains a large amount of potassium ions, which produces greater irritation, so the pain is more obvious when injecting. 2. Penicillin particles are large and not easy to be absorbed: Penicillin particles are large and difficult to dissolve in water, so it is easy to be clogged with needle when it is injected intramuscularly, therefore, it needs to be rapidly injected into the human muscle, and due to the large amount and slow absorption, the pain in this process is relatively strong, and the pain lasts for a long period of time. In order to reduce the pain, the clinic can heat the penicillin injection drug appropriately. Clinics can sometimes change the route of administration, or lidocaine formulated penicillin injection to reduce pain, if the injection site appears swollen lumps and other need for hot compresses or anti-infective treatment, it is recommended that patients go to the regular hospital for injection, to avoid delaying the condition.