ContextThe modern version of the Hippocratic Oath requires doctors to swear that they will apply, for the benefit of the sick, all measures that are required, avoiding the twin traps of overtreatment and therapeutic nihilism. This paper explores the magnitude of the problem of overtreatment and undertreatment and the potential sources of these treatment errors. MethodsWe undertook a narrative review of the literature on errors in treatment associated with flaws in doctors' judgements and present evidence from research into clinical reasoning and from psychological research into decision making. Based on evidence from these two research fields, we explored the possible reasons why doctors erroneously withhold or unnecessarily administer treatments. ResultsVariation in treatment has been documented, even with similar clinical presentations under a variety of conditions, suggesting that overtreatment and undertreatment actually occur, with adverse effects for patients. Both types of error have been demonstrated, even when the doctor arrived at the correct diagnosis. They may be associated with the influence exerted on doctors' treatment judgements by factors that are unrelated to the specific problem, such as patients' socio-demographic characteristics and the doctor's practice culture. Doctors are also subject to commission bias and to omission bias, which have been demonstrated to occur in several domains. Such biases lead doctors to administer unnecessary treatments or to withhold required treatments due to anticipated regret. Little is known about cognitive processes underlying doctors' treatment decisions, but mental representations of diseases that provide the basis for diagnostic reasoning are also probably used for treatment judgements. ConclusionsDoctors are at risk of falling into the twin traps of overtreatment and therapeutic nihilism. Further research should explore how to avoid these traps, but it may require deliberate reflection on problems to be solved to counteract the influence of factors that are beyond the patient's problem. Discuss ideas arising from the article at discuss'