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FIGURES COURTESY OF THE AUTHORS. 14 Pharmaceutical Technology ® | BioPharm International ® Trends in Manufacturing May 2025 PharmTech.com COMPOUNDING affect the accuracy of calculations are described. Topics address small-molecule compounding; large-molecule biologic drugs are not compounded (5,6). Part one in the series addresses terminology, regula- tions, and labeling respective drug types. Subsequent topics will include numeric calculations, moisture, material losses, and related topics that build on prior publications (7,8). Drug calculations begin with the medication order—individual prescription, hospital order, 503B outsourcing batch, pharma clinical supply request, research formulations, or other request for a compounded preparation. The medication order iden- tifies the desired active moiety by proprietary or generic name along with the duration of treatment. Types of drugs used in compounding are identified. Compound- ing personnel must select an appropriate drug for com- pounding—there may be multiple choices—and calcu- late its amount. Drug selection must integrate patient considerations, physicochemical properties, material handling, and other applicable considerations. After drug selection, calculations are executed; each drug type requires different calculations. Post calculations, labeling must clearly define the compounded prepara- tion. Figure 1 shows an overview of calculations-related activities from medication order initiation through final dosage form dispensing and/or shipping; specific part one discussion topics are highlighted in red. Terminology Drugs. Drugs are known by a variety of terms depend- ing on regulatory agency, country, site vernacular, and other considerations. Some of the more familiar terms include API, active substance, drug substance, bulk drug, bulk drug substance, active, bulk, and so on. "API" terms are more common in pharma indus- tr y (e.g., International Council for Harmonisation (ICH) Q7A Good Manufacturing Practice Guidance for Active Pharmaceutical Ingredients [9]); "bulks" lan- guage is common in compounding discussions (e.g., Bulk Drug Substances Used in Compounding Under Section 503A of the US Food, Drug and Cosmetic Act (FD&C Act) [10]; Bulk Drug Substances Used in Com- pounding Under Section 503B of the FD&C Act [11]). Active moiety. The active moiety is the part of the drug responsible for pharmacologic activity. In the- or y, the active moiety excludes molecule portions such as esters, counterions, complexes, chelates, clathrates, and waters of hydration that do not con- tribute to dr ug activit y. The active moiet y is con- tained within the drug molecule; it may be part or all of the molecule (e.g., diazepam is generally con- sidered to be the active moiety and active drug in Valium). The distinction between active moiety and active drug is critical for understanding calculations and labeling in salt drugs. FIGURE . Drug calculations process.