Whether mother tongue should be used in EFL classroom or not is a controversial issue and has not yet reached a consensus among teachers and researchers. While some argue that the use of mother tongue inhibits language learning, others claim that it saves time and energy for both language teachers and students and enhances mutual understanding between them. Although a number of studies explore the use of mother tongue in EFL classrooms, few have been conducted to investigate how often teachers in non-English major classes code-switch, that is, change from English to mother tongue and why they do that. In such a context, the current study examined the use of codeswitching by teachers in EFL classroom in a medical college in Vietnam by means of classroom observations and voice recording analysis. The findings revealed that in teaching English to nursing students in this medical college, the teachers did codeswitch to a great extent for the main purpose of enhancing their students’ English comprehension and competence. Suggestions are proposed to raise EFL teachers’ awareness on how and when to code-switch so that their teaching can be optimized.