Introduction: Clinical guidelines have recommended a 1-year trastuzumab regimen as standard care for early human epidermal growth factor receptor 2 (HER2)-positive breast cancer; however, this recommendation can have a dramatic impact on total drug expenditures in middle-income countries (MICs). We performed a cost-effectiveness analysis from the Iranian healthcare perspective to find an optimum duration of trastuzumab use in Iran. Method: We compared four treatment strategies comprising chemotherapy and varying durations of trastuzumab use (no trastuzumab, 6, 9 months, and 1 year), and a Markov model and probabilistic sensitivity analysis were used to estimate the costs and effects of the strategies. We then examined the cost effectiveness of the strategies at different willingness-to-pay (WTP) thresholds and ages at onset of treatment. Results: Incremental costs (versus no trastuzumab) were €8826 (6 months), €13,808 (9 months) and €18,588 (12 months), while incremental quality-adjusted life-years (QALYs) were 0.65 (6 months), 0.87 (9 months) and 1.14 (12 months). At a threshold of 3 × gross domestic product (GDP)/capita (€21,000/QALY) and for patients younger than 59 years, the 6-month protocol was most likely to be cost effective (probability of 42%). At a threshold of 4 × GDP/capita (€28,000/QALY), the 6-month and 1-year regimens were essentially equal in cost effectiveness (37 and 35%, respectively). At this WTP threshold, the 6-month and 1-year regimens were optimal strategies only for patients up to 66 and 44 years of age, respectively. Conclusion: In contrast to clinical guidelines, 6 months of trastuzumab may be the most cost-effective option for Iran. The lower absolute WTP threshold and lower life expectancy compared with high-income countries are two crucial parameters in the cost effectiveness of interventions in MICs. It is therefore necessary to strike a balance between maximum population health and maintaining affordability in these countries.