The Four-Factor Test: How FDA Evaluates a Bulk Drug Substance (21 CFR 216.23)

The regulatory frame

When FDA evaluates whether a nominated bulk drug substance should be recommended for the Section 503A Bulks List, career staff apply a published balancing test under **21 CFR 216.23(c)**. The regulation directs consideration of four factors:

1. **Physical and chemical characterization**

2. **Historical use in compounding**

3. **Evidence of clinical effectiveness**

4. **Safety concerns**

No single factor is a standalone “pass/fail” switch. Staff weigh them together and state a conclusion in briefing documents prepared for advisory meetings.

Factor 1 — Characterization

Can the substance be reliably identified and described? Inconsistent naming across commercial products, uncertain composition, or weak analytical definition can weigh against listing. FDA staff have emphasized that without a stable understanding of composition, long-term scientific assessment of a bulk drug substance is difficult.

Factor 2 — Historical compounding use

Has the substance been compounded in a way that FDA can document and evaluate? Historical use does not equal proof of effectiveness, but it informs the agency’s understanding of how the substance has appeared in compounding practice.

Factor 3 — Effectiveness evidence

FDA looks for clinical evidence adequate to support nominated uses. Briefing packages for recent peptide nominations described gaps ranging from “no human clinical data” for some substances to small or poorly controlled studies for others. Staff may also decline to evaluate nominated indications when supporting studies are not identified.

Factor 4 — Safety concerns

Safety analysis can include adverse-event databases, pharmacokinetic gaps, immunogenicity or aggregation concerns, and whether FDA-approved alternatives already exist for a nominated use. External status flags (for example, anti-doping prohibitions) may also appear in staff discussion as appropriateness context.

How to read briefing documents

When FDA staff recommend against listing, they typically walk through all four factors and then state that the balance weighs against addition. When an advisory committee votes differently, that vote still sits on top of — and does not erase — the staff analysis. For educational readers, the four-factor test is the clearest map of *how* FDA frames these decisions.

*For research use only. Not for human or animal consumption.*