Which abbreviation indicates that a medication is to be given as needed?

The abbreviation that indicates a medication is to be given as needed is "prn," which stands for the Latin phrase "pro re nata." This phrase translates to "as the situation arises" or "as needed." It is commonly used in medical settings to instruct healthcare professionals about the administration of medications that are not given on a regular schedule but rather as the patient's condition requires.

In contrast, the other abbreviations have specific meanings: "pm" typically refers to "post meridiem," indicating the time of day (afternoon or evening), while "q" denotes the frequency of administration, meaning "every" (like in q4h for every four hours). "OD" is an abbreviation for "once daily," which specifies that a medication should be taken once each day. Understanding these distinctions is crucial for accurate medication administration and patient care.