Economic evaluations play a crucial role in health resource allocation by assessing the costs and effects of various interventions. However, existing methodologies often overlook significant differences related to sex and gender, leading to a 'blind spot' in understanding patient heterogeneity. This paper highlights how biological and social factors influence costs and health outcomes differently for women, emphasising the need for a more explicit consideration of these differences in economic evaluations to ensure efficient and equitable resource allocation. The paper is structured to first outline how sex and gender factors impact costs and outcomes. It then identifies biases in current economic evaluation methods and practices, using real-world examples to illustrate the implications of these biases on policymaking and health equity. Notably, we argue that neglecting gender considerations can lead to inefficiencies and inequities in healthcare resource distribution. Key areas of gender bias include the estimation of productivity losses, quality of life variations and the secondary household effects of interventions. The analysis reveals that women often face higher healthcare costs and experience different health outcomes due to systemic biases in treatment and care. The paper concludes with practical recommendations for analysts, decision makers and research funders, advocating for the integration of sex and gender-responsive methodologies in health economic evaluations. Ultimately, this work calls for a paradigm shift in health economics to better reflect the complexities of sex and gender and improve health outcomes for all.