Background: Hospital formularies are used to encourage the use of safe, effective and most affordable medications. Institutions need to make provisions for non-formulary medicines (NFM) due to the dynamic nature of diseases and their management. The aim of this study was to describe the patterns of non-formulary medicine prescriptions at the Nairobi Hospital, the reasons for their purchase as well as the duration taken to avail them. Methods: A descriptive review of all the non-formulary medicine prescriptions from January 2021 to June 2022. The medicines were listed and categorized according to the WHO Anatomical Therapeutic Chemical (ATC) classification system. Correspondence between pharmacy and procurement was reviewed to understand the reason for the requests and the duration it took to avail the medicines. Results: A total of 183 NFM were purchased, with a general increase in the number from January 2021 to June 2022. Vitamins, Mineral supplements and General nutrients accounted for 41 (22.4%) of the NFM. Dermatologicals 27 (14.6%), Genito-urinary system drugs and sex hormones 20 (10.9%), Ophthalmologicals 14 (7.6%) and Antineoplastic and Immunomodulating agents 12 (6.6%) were also frequently purchased out of formulary. The main reasons for NFM purchases were: having no therapeutic equivalents in the hospital formulary 72 (39.3%) and prescriber or patient preference 69 (37.7%). It took a median (IQR) of 4 (2 -7) days for the pharmacy to avail these drugs; with 18.6% being availed in 1 day and 55.2% taking more than 3 days. For the NFM where no alternative was available in the hospital formulary, sales amounted to USD 63,362 which was 79.1% of the value of all the NFM sales. Conclusion: There's a need to regularly update the hospital formulary and to emphasize to the prescribers the importance of adhering to it, as much as possible.