The objective of this study was to analyze the costs and benefits of the intensive adverse products reactions (APRs) monitoring program of inpatients in medical wards at Nakhon Ping Hospital, Chiang Mai, Thailand. The data were retrospectively collected from inpatients who had APRs during admission period from November 16, 2004 to March 13, 2005. Products included were drugs, electrolyte solutions, bloods and blood derivatives.  Only direct metical costs were considered using provider’s perspective. The results showed that found there were 1,407 admitted patients during the study period. Adverse products reactions were found in 31 patients. Of those, three patients had two APRs. Therefore, a total of 34 APRs were found yielding the APRs incidence rate of 24 per 1,000 inpatients. Of the APRs found, 20 were reported after the symptoms had begun, but the remaining APRs were preventable. Drugs were the main causes for APRs (94.12%). Cost of intensive APRs monitoring was US$1,426.37 and included US$939.44 for labor costs, US$12.5 for material costs, US$29.07 for capital cost and US$4,040.85. Net benefit and B/C ratio were US$2,614.48 and 2.84 respectively. The results of sensitivity analysis represented that net benefit and B/C ratio were increased when duration of digoxin intoxication was prolonged. However, net benefit and B/C ratio were decreased when pharmacist labor cost was increased. Results indicate that intensive APR monitoring is a cost beneficial program and should be continuously implemented.