Clinical mastitis of dairy cows is a visible inflammation of the udder, which is usually caused by bacteria and treated with antibiotics. Although pressure is increasing to reduce antibiotic usage in livestock in the European Union, feedback from the field suggests that clinical mastitis treatment is frequently repeated after the initial per-label treatment, thereby extending treatment duration. The aim of this study was to explore the social factors influencing farmers' decision-making on the duration of antibiotic treatment of clinical mastitis. In total, 38 dairy farmers in the Netherlands (n=17) and Germany (n=21) were interviewed in a qualitative semi-structured way. Extended treatment was defined as any treatment longer than that given in label directions. Of the 38 farmers, 30 reported routine and 7 occasional extended antibiotic treatment. The interviewed farmers were sensitive toward social norms of other farmers and recognition for good stockmanship. Extended treatment is perceived as part of the social norm of "being a good farmer." The participants' perception was that mastitis is not treated "thoroughly" if clinical symptoms were still visible at the time of cessation of treatment, because it may persist or recur. As a result, treatment was frequently extended by repeating the initial label treatment. Farmers, specifically the more "cow-oriented" farmers, expressed insecurity on how to treat mastitis effectively. This insecurity made them more sensitive to comply with other farmers' injunctive ("what ought to be") and descriptive ("what is done") norms and the perceived veterinarians' informational norm that extended treatment is better, resulting in an approved social norm. Social approval reduces the insecurity of being perceived as a poor farmer; thus, extended treatment is emotionally rewarded. This social reward apparently outweighs the higher costs of more waste milk and more antibiotic usage. Perceived positive reference groups with whom the farmer identifies and regularly communicates face to face, such as other farmers, the herd veterinarian, and other farm advisors, confirm the farmer's judgment on extending treatment and influences him or her toward socially accepted behavior. Society was the most negative reference group, barely influencing farmers' decision-making on treatment. The emotional gap between farmers and society is large and probably difficult to overcome. Legislation may reduce antibiotic usage, if doable and controllable. Evidence-based information on treatment efficacy or practical on-farm decision support indicating when to end treatment may be able to change social norms of "thorough" treatment, especially when communicated by a positive reference group such as veterinarians. Because prudent antibiotic use is hindered by perceived subjective norms on optimal duration of antibiotic treatment, more research is needed, particularly on the optimal duration of antibiotic treatment of specific pathogens as related to cure and recurrence of clinical mastitis.