Interventions performed by 89 community pharmacists in 5 states to correct the prescribing problems they identified on new prescription orders were documented by trained observers. Pharmacists intervened to resolve a prescribing-related problem in 623 (1.9%) of 33,011 new prescription orders that were screened and dispensed during the study period. A panel of three expert evaluators concluded that 28.3% of the prescribing problems identified during the study could have caused patient harm if the pharmacist had not intervened to correct the problem. The rate at which pharmacists identified prescribing problems was negatively related to the number of prescriptions they dispensed per hour, suggesting that in pursuing distributive efficiency, some pharmacists may be exceeding their safe dispensing threshold. The authors recommend that the interprofessional system of oversight and verification (i.e., "checks and balances") in the delivery of pharmaceutical care in the community setting should be maintained and strengthened.