Thesis Title: Evaluation of hyperkalemia associated with intravenous co-trimoxazole in hospitalized patients at Sultan Qaboos University Hospital. Background: Co-trimoxazole is a combination of two antimicrobial drugs, (Trimethoprim and Sulfamethoxazole), that is used to treat a wide variety of infections. Hyperkalemia is one of the seriously life-threatening electrolyte disturbances, that is also associated with the use of trimethoprim. The risk of hyperkalemia is increased if co-trimoxazole is concurrently prescribed with drugs that can increase plasma potassium concentration. Aim: To determine the incidence of hyperkalemia and its risk factors among hospitalized patients receiving intravenous co-trimoxazole at Sultan Qaboos University Hospital in Muscat, Oman. Materials and Methods: A retrospective observational study involving patients who received intravenous co-trimoxazole. Subjects were identified through a list generated from a computerized pharmacy system in the period between January 2010 and December 2020. The patients' demographic and clinical characteristics including serum potassium level, duration reaching the maximal serum potassium level, dosage, and concomitant medications were retrieved from electronic medical records. Hyperkalemia was defined as a serum potassium level greater than 5.1mmol/L. Data was analyzed using both descriptive and inferential statistical tests. Results: 420 patients participated in this study. The median age of the patients was 51 year and 55.5% were males. Hyperkalemia associated with co-trimoxazole was observed in (40.2%) of the patients. Around (44.2%) of the patients who experienced hyperkalemia received a high dose of co-trimoxazole. Hyperkalemia occurred after the 5th day from the beginning of co-trimoxazole treatment. The logistic regression analysis showed no relation between hyperkalemia and age (adjusted odds ratio AOR 1.054; p=0.840), gender (AOR 1.167; p=0.471), dose (adjusted OR 0.779; p=0.251), and use of concomitant medications (ACEi AOR 1.054; p=0.840, ARBs AOR 0.564; p=0.734, β -blockers AOR 0.986; p=0.963, Potassium supplement AOR 0.590; p=0.175, NSAID AOR 0.842; p=0.684, spironolactone AOR 0.748; p=0.629, heparin AOR 0.822; p=0.382,CNI AOR 1.537; p=0.406). Conclusion: Co-trimoxazole is associated with a high incidence of hyperkalemia in adult patients. There are no association between hyperkalemia and risk factors due to the limited sample size in our study. Potassium levels should be monitored in the first week of cotrimoxazole treatment