This article will discuss the implications of nurse prescribing in mainstream primary health care and its impact on the fields of mental health and learning disabilities. Complexities and issues which require serious consideration by those nurses wishing to pursue such a specialist and extended role will be discussed in relation to these practice areas. Titchen's (1998) critical companionship model will be illustrated as an example of one framework for clinical supervision. This is to allow the processes, competencies and contextual issues to be explored by both novice and expert prescribers. The article concludes that for safe, effective and competency-based practice, clinical supervision also assists in mediating the professional and political aspirations in support of supplementary nurse prescribing.