This study aimed to (1) elaborate on microblogging-based instruction, which was built on social media platform X and designed based on the four phases of the design-based research (DBR) framework (Amiel and Reeves in J Educ Technol Soc 11(4):29-40, https://bibliotheek.ehb.be:2090/stable/jeductechsoci.11.4.29, 2008); (2) put microblogging-based instruction into practice in a medical English for Specific Purposes (ESP) course; (3) examine students' perceptions of the pedagogical value of X platform usage on their learning experiences; and (4) investigate how well microblogging-based instruction enhances the students' specialised-language performance. On the basis of the four phases of the DBR framework, a study was conducted with students studying a medical ESP course to identify the problems in the teaching practices used in ESP courses. To improve the identified problems, following the second phase, social media platform X was used to supplement ESP face-to-face instruction. Three tasks were designed for the students to do on X. These tasks were implemented and assessed on 19 EFL first-year premedical undergraduate students over 16 weeks at a university in the Kingdom of Saudi Arabia. Data collected from the students included a copy version of the ESP final test, a questionnaire, and semi- structured interviews. The results revealed that X-based instruction improved students' ability to use medicine-specialised language, offered opportunities for students to become more familiar with medicine-specific terms, perceived ability in using writing and reading skills, collaborative learning, and generated thoughtful discussions outside the confines of the classroom. However, the students' inadequate knowledge of the specialised subject, the stress of making X mandatory, and log- in overload remain key challenges against the effective appropriation of X use in an ESP context.