Pharmacological information has been organized and encoded in rules and frames for systematic retrieval. This includes: delineation, definition, and hierarchical subdivision of mechanisms responsible for drug interactions; division of pharmacological agents into a hierarchy of subclasses to allow for defining interacting drugs by classes as well as by specific agents; correlation of drug classes and specific drugs with mechanisms by which they may be involved in drug interactions. This information, accessible through a natural language-like and menu driven interface, allows clinicians to know what may happen when two drugs are used together, why, what can be done to alleviate detrimental interactions, and what related drugs may also be involved in similar interactions.