In this study, the influence of the capillary inner diameter (id) on the efficiency and sensitivity of a capillary zone electrophoresis separation was investigated. Four local anaesthetic drugs (lidocaine, prilocaine hydrochloride, procaine and tetracaine) were separated with a validated method using capillaries of different id. The separation parameter N and the resolution of the critical peak pair were monitored. It was found that N increases in tighter capillaries, while sensitivity decreases indicated by the increased detection limit for lidocaine. This loss in sensitivity can partially be compensated by loading more sample into the capillary by means of an increased injection time and pressure. However, when it comes to the evaluation of the drug quality according to a monograph in the European Pharmacopoeia, we cannot recommend to vary the capillary id in order to meet the system suitability criteria.