PRN means when necessary, and is generally used in long-term medical advice, in long-term medical advice generally includes BID, QD, TID and other medication instructions, BID is two times a day, QD is one time a day, TID is three times a day means. If a PRN is used after a medical order, it means when necessary, which means that after the doctor has given the order, perhaps according to the patient’s condition, some treatment can be done without the doctor’s permission when needed, such as oxygen PRN, the nurse will put the patient on oxygen when necessary according to the patient’s oxygen saturation and the patient’s breathing condition.