Many terminologies exist for the form of drugs--i.e. tablets, capsules, sprays, suppositories, etc. However, they have surprisingly different content. To communicate medication messages effectively, a mechanism is needed to translate between these existing terminologies. An ontological approach, based on techniques developed by OpenGALEN, has been used to build a drug form terminology for HL7 version 3. It integrates existing terminologies from commercial drug information vendors and regulatory authorities, and provides a framework for translating between them. To date, term sets have been included from First DataBank, the FDA, Multum and Micromedex, to produce a terminology of 820 concepts. The approach is made practical by distributing the knowledge engineering effort to volunteers with experience of the domain, and then integrating the knowledge into a logically consistent classification.