OBJECTIVES:  Clinical guidelines around the world have recommended a one-year trastuzumab regimen as standard care for early HER2-positive breast cancer (BC). However, this recommendation can have a dramatic impact on total drug expenditures in middle-income countries (MICs) if trastuzumab were to be reimbursed. This study provides an updated cost-effectiveness analysis from the Iranian healthcare perspective and investigates a proposed duration of trastuzumab use in Iran. METHODS:  We compared four treatment strategies comprising chemotherapy and varying durations of trastuzumab use (0/6/9/12 months). A Markov model and probabilistic sensitivity analysis were used to estimate the costs and effects of the strategies. Cost data were obtained from a recent real-world cost analysis. We then examined the cost-effectiveness of the strategies at different willingness-to-pay (WTP) thresholds (€21,000/QALY or 3×GDP/capita, based on WHO-CHOICE; €28,000/QALY or 4xGDP/capita). RESULTS:  Incremental costs (vs. no-trastuzumab) were €9,171 (6-month), €14,211 (9-month) and €19,067 (12-month) while incremental effectiveness were 0.65 (6-month), 0.86 (9-month) and 1.13 (12-month). At a threshold of 3×GDP/capita, the 6-month protocol was the most cost-effective in 41% of simulations, followed by one-year (19%), 9 months (20%) and no-trastuzumab (20%). When the threshold was increased to 4×GDP/capita, the 6-month and one-year regimens were essentially equal in cost-effectiveness (36% and 35%, respectively), followed by 9 months (20%) and no-trastuzumab (9%). The relative budget impact of trastuzumab use (12-month vs 6-month) was estimated at €40 million/year in Iran. CONCLUSIONS:  In contrast to clinical guidelines, 6 months of trastuzumab may be the most cost-effective option for the Iranian healthcare setting. However, a higher threshold leads to one-year trastuzumab use being just as cost-effective as 6 months. It is therefore important for policymakers and other stakeholders in MICs to determine which factors should drive trastuzumab reimbursement decisions. Implementation of widely accepted clinical guidelines required a price reduction or acceptance of a higher WTP threshold.