<p>The Internet of Medical Things (IoMT) is a network made from various medical infrastructures and sensors connected using wireless body area networks (WBANs). Time-sensitive medical information is critical for IoMT applications. Data are crucial for making data-driven decisions and for predictive analytics. Considering the sensitive nature of data in the healthcare industry, data security is essential. Considering the resource-constrained nature of IoMT, an efficient authentication protocol is mandatory to achieve privacy in message transmission. The paper proposes a lightweight authentication scheme to secure communication in smart healthcare systems in IoMT. The approach minimizes communication delays by reducing the dependency on a central authority and alleviating the computational burden on sensors and network devices, which are key drawbacks of conventional methods. The formal analysis of the authentication scheme is performed using the Scyther tool, and an informal analysis of the scheme is also done. The analysis shows that the system is resistant to various attacks. The overhead costs of the system were analyzed and compared to recent lightweight authentication schemes. The LASH protocol decreases the communication cost by nearly 48 percent compared to existing authentication methods, whereas the computation cost of the scheme is 4.532, which is much lower than the recent schemes. Consequently, the proposed scheme ensures secure authentication while minimizing computational and communication expenses.</p>