We describe here the prescription algebra and its implementation for medication authoring tools. The tools are parts of medication use process, which consists of prescription entry systems, medication authoring tool, medication scheduling specification, medication scheduler, and programmable pill dispenser. A medication authoring tool aids the pharmacists to collect and integrate prescriptions, to verify drug-drug interactions amongst prescriptions one took for the prescribed duration, and to generate the scheduling specifications for pill dispensers. We design a prescription algebra for medication authoring tool to correctly complete its work. We have implemented the platform-independent medication authoring tool using JAVA. The screen snapshot are shown in the paper.