Aims To examine the nature, frequency and determinants of prescription modi®cations in Dutch community pharmacies. Methods A prospective case-control study comparing modi®ed prescriptions with nonmodi®ed prescriptions was carried out in 141 Dutch community pharmacies. 2014 modi®ed prescriptions (cases), collected in the selected pharmacies on a predetermined day in a speci®c period (25th February until 12th March 1999) and 2581 nonmodi®ed prescriptions (controls) randomly selected on the same day were studied. The nature and frequency of prescription modi®cations and patient, drug and prescriber related determinants for a modi®ed prescription were assessed. Results The overall incidence of prescription modi®cations was 4.3%, with a mean of 14.3 modi®cations per pharmacy per day. For prescription only medicines (POM) the incidence was 4.9%. The majority of POM modi®cations concerned a clari®cation (71.8%). In 22.2% a prescription could potentially have had clinical consequences when not altered; in more than half of the latter it concerned a dose error (13.7% of all cases). POM prescriptions of patients of 40±65 years had a signi®cantly lower chance of modi®cation compared with those of younger people (OR=0. 74 [0.64±0.86] .57]) gave a higher probability of prescription modi®cations. When a GP had no on-line access to the computer of the pharmacy the chance of a modi®cation was also higher (OR=1.61 [1.33-1.94]). Multivariate analysis revealed that a nonprinted prescription was the strongest independent determinant of prescription ), remaining so after adjustment for GP computer link to the pharmacy and for type of prescriber. Conclusions At least 30% of Dutch community pharmacies corrected 2.8 POM prescriptions per pharmacy per working day, which could potentially have had clinical consequences if not altered. If the study sample is representative for The Netherlands, Dutch community pharmacies correct a total of approximately 4400 of these prescriptions per working day. Using computerized systems to generate prescriptions is an important strategy to reduce the incidence of prescription errors.

Based upon our research, all Dutch community pharmacies compound more than 13,000 medicines per day (2.3% of all prescriptions). They consist mainly of dermatological preparations. Younger children (< 12 yr) receive a significantly higher rate of compounded medicines than other people. At least 1.2 compounded prescriptions per pharmacy per day have a specific pharmaceutical care reason according to the pharmacists.