Pharmacists in the UK were granted supplementary prescribing rights in 2003, subject to further training. Most of those now qualified are practising in general medical practices and hospitals. This qualitative study explores GP and pharmacist perceptions of the introduction of pharmacist supplementary prescribing, focusing on the consequences for professional boundaries, power relations and knowledge. GPs have delegated some routine work in specific chronic conditions, and a limited amount of decision making, to pharmacists, (albeit within tightly controlled boundaries). But diagnosis has remained firmly in the hands of GPs; work being delegated in areas where the pharmacist was minimally required to exercise diagnosis and clinical judgement. GPs have continued to exercise control over inter‐professional boundaries.