Prescribing drugs for the treatment of medical conditions is a very common activity for a doctor. Prescribing has enormous economic importance. Costs are rising quickly and there is an urgent need for doctors to have easy assess to advice about the cheapest, most effective therapy. On average 80% of GPs in the UK use a computer for their medical work and the figure is rising rapidly. Currently available systems provide only very simple checks and reminders. More sophisticated advice is provided by our prototype program. The program uses logic engineering to give advice, based on simple protocols for prescribing, tailored both to the condition being treated and the individual patient. The essential logical elements of the prescribing decision are discussed. These simple prescribing protocols may be the final common pathway for prescribing advice from many, more complex protocols for recommendations for drug treatment.