Because preventive care is a critical step in promoting public health, medical professionals provide health education in order to inform the public how to avoid diseases. However, in a multilingual society such as Taiwan, where Western medical discourse is carried on mainly in Mandarin (the official language) and English (a foreign language and the lingua franca of Western medicine), the issue of how medical professionals communicate their knowledge to the lay public, especially to elderly patients who are monolingual in the local dialect of Southern Min, is a pressing concern. This paper examines two osteoporosis education talks conducted by a male visiting physician at a teaching hospital in southern Taiwan. The analysis focuses on the speaker’s choice of language and lexical synonyms in conveying the two key concepts of ‘bone quality’ and ‘non-solid’, i.e., osteoporosis. By applying Ferguson’s high and low varieties (1972: 234) and Tsai’s four principles in quantifying lexical formality (2008), it is observed that, due to the lack of an established writing system, Southern Min is hardly used in the written slides of the speaker’s presentation, and in fact occurs even less often than English does. Instead, Chinese characters representing Mandarin are the dominant language used in the written messages. While this form of presentation might cause comprehension problems for illiterate and monolingual speakers of Southern Min, this communication problem is offset by the speaker’s preference for Southern Min, the vernacular language, in the spoken form. Further discourse analysis leads to the arguments that (1) the speaker’s preference for linguistic expressions with less formality in the spoken discourse is a strategy in providing comprehensive and easy-to-access medical knowledge for the lay audience, (2) the de-formalization process of medical jargon moves from the higher codes to the lower ones in the discourse flow, and (3) the complementary distribution of higher codes on the written slides and lower codes in the spoken form facilitates the speaker’s task of delivering medical knowledge. The pedagogical implications of this research provide a practical guideline for medical professionals with regard to promoting the public’s medical literacy.