Drug prescription and administration processes strongly impact on the occurrence of risks in medical settings for they can be sources of adverse drug events (ADEs). A properly engineered use of information and communication technologies has proven to be a promising approach to reduce these risks. In this study, we propose PHARMA, a web information system which supports healthcare staff in the secure cooperative execution of drug prescription, transcription and registration tasks. PHARMA allows the easy sharing and management of documents containing drug-related information (i.e., drug prescriptions, medical reports, screening), which is often inconsistent and scattered across different information systems and heterogeneous organization domains (e.g., departments, other hospital facilities). PHARMA enables users to access such information in a consistent and secure way, through the adoption of REST and web-oriented design paradigms and protocols. We describe the implementation of the PHARMA prototype, and we discuss the results of the usability evaluation that we carried out with the staff of a hospital in Florence, Italy.