This study aims to measure the longitudinal impact of a 60-hour adjunct instruction course, involving L2 content-based teaching with a systematic focus on form, on students' grammatical and lexical knowledge and on their receptive skills (i.e. reading and listening) in a foreign language higher education context. The participants were 52 university students enrolled in two different strands of the same Dentistry degree: an English-Medium Instruction (EMI) group, with no explicit L2 teaching/learning objectives, (n = 25) and an L1 group (n = 27) (Catalan/Spanish). In the L1 group, students had to read articles and teachers' powerpoint presentations in the L2. Quantitative data collected by means of a battery of tests over 16 weeks indicate that adjunct instruction leads to statistically significant improvement in overall L2 language scores for all students alike. Nonetheless, results show that adjunct instruction tends to bring larger L2 grammatical improvement and significantly higher gains in receptive skills to Dentistry students who receive minimal English Foreign Language exposure (i.e. the L1 strand) than to those with massive exposure to English (i.e. the EMI strand).