Primary care prescribers must cope with an increasing number and complexity of considerations. Prescribing decision support systems (DSS) have therefore been developed to assist prescribers. Previous studies have shown that although there is wide variance in the different DSS available within primary care, barriers and facilitators to uptake remain. The Drug Synonyms function ('Synonyms') is a DSS inherent in the commercial electronic medical record system EMIS. Synonyms functionality has been further developed by the NHS Greater Glasgow and Clyde (GG&C) Central Prescribing Team to promote safe and cost-effective prescribing; however, it does not support the collection of usage data. As there is no knowledge on the uptake nor on the perceived effect of using Synonyms on prescribing, quantitative and qualitative analyses of Synonyms usage are required to ascertain the impact Synonyms has on primary care prescribers, which will influence the continued maintenance and/or future development of this prescribing DSS. To determine the uptake of Synonyms and explore users' perceptions of its usefulness and future development. An exploratory sequential mixed-method observational study using quantitative questionnaires, followed by semi-structured interviews with primary care prescribers within NHS GG&C. An electronic questionnaire (Questionnaire 1) accessible across 218 EMIS-compliant NHS GG&C GP practices ascertained Synonyms uptake by determining whether prescribers were aware of the DSS, whether they were aware of it and whether they used it. Prescribers who were aware of and used Synonyms were asked to opt in to participating further. This involved answering a second electronic questionnaire (Questionnaire 2), with the option of taking part in an additional one-to-one interview, to investigate their use and perceptions of Synonyms. Questionnaire 1 was completed by 201 respondents from 43.1% of eligible GP practices: 186 (92.5%) respondents were aware of Synonyms, of whom 163 (87.6%) had used it and 155 (83.3%) continued to use it. Questionnaire 2 was completed by 104 respondents: 90 (86.5%) indicated that Synonyms informed or influenced their choice of drug prescribed; 94 (90.4%) reported that Synonyms changed their prescribing choice towards medication on NHS GG&C formulary, and 104 (100%) reported that they trust Synonyms. Six interviews generated suggestions for improvements, mainly extending the clinical conditions listed. Most respondents were aware of and continued to use Synonyms. Respondents perceived Synonyms to influence prescribing choices towards local formulary medicines and improve adherence to local prescribing guidelines. Respondents trusted the DSS, but there is potential to increase awareness and training amongst non-users to encourage usage. Potentially, the NHS GG&C Synonyms function could be utilised by other health boards with supportive clinical systems. © 2020 Royal Pharmaceutical Society.