OBJECTIVES: The pharmacist intervention system started at East Province of Ministry of Health in 2013. The pharmacist interprets and prevents all drug related problems. The objective of this study was to explore pharmacist intervention and prevented medication errors at Pediatrics, Obstetrics and Gynecology hospital in East province, Saudi Arabia. METHODS:  It is a cohort study of 12-months, 2015 of pharmacist intervention and prevention medication errors. The system established at 500-bed Pediatrics, Obstetrics and Gynecology Hospital in 2013. This system was a part of medication safety program. The hospital had medication safety officer with medication safety committee. Any intervention should document through medication error form. The structure consisted of patient information, the sources of medication errors, time of mistakes. The type of medication errors, description of errors, causes of errors, recommendation to prevent the errors, and the outcome of medication errors by using National Coordinating Council (NCC) for Medication Error Reporting and Prevention (MERP) system. RESULTS:  The total number of prevented medication errors were 1654 in 827 prescriptions. The majority of medication errors committed through physicians followed by nurses. The most error stopped with Pediatrics (1-month to 6 years) followed by young adults (18-40 years). The majority type of mistakes was prescriber-related 1216 (73.52%) followed by patient-related errors 426 (25.75%). The outcome of medication errors were near miss 1651 (99.82%). The most medications prevented errors were Paracetamol syrup, Iron tablet, Folic acid tablet, and Calcium tablet. There were three errors of high-risk medications were avoided included Insulin, Enoxaparin, and Heparin CONCLUSIONS:  The pharmacist plays a significant role in preventing medication errors, especially in pediatrics populations. The necessity of pharmacist provides basic medication safety education to all health care providers. Expanding pharmacist role in preventing medication errors associated with patient safety and avoid the un-necessary cost