Although antibiotics are prescription-only medicine in Indonesia, they can be purchased without prescription. This qualitative study elicited beliefs about nonprescribed antibiotics use informed by the theory of planned behavior to develop a questionnaire for an expanded theory of planned behavior survey. Twenty-five (N = 25) adults with experience of using nonprescribed antibiotics were interviewed. Content analysis was applied. Participants reported that the use of nonprescribed antibiotics was advantageous in term of saving time and money and of reducing the number of medicines that need to be purchased, in contrast to a perception of what occurs with medical prescriptions. Potential adverse effects, poor health outcomes, and antimicrobial resistance were the perceived disadvantages. Facilitators of such use were the availability of over-the-counter antibiotics and successful experience in using antibiotics. Medication for children was the perceived barrier to such use. Family members and friends, especially those with health education background, approved of such use.