Explainer

What is the difference between a 503A and a 503B compounder?

Short answer · as of 2026-10-05

A 503A compounder is a licensed pharmacist or physician, usually in a state-licensed pharmacy, compounding for an identified patient on a valid prescription, overseen mainly by the state board of pharmacy. A 503B compounder is an outsourcing facility that registers with FDA, follows current good manufacturing practice, and may or may not have patient-specific prescriptions. Neither makes a compounded drug FDA-approved.

By Peptide Industry Gossip · Published · General explanation, not legal or medical advice. Disclaimer.

Who they are

  • 503A: a licensed pharmacist in a state-licensed pharmacy or federal facility, or a licensed physician.
  • 503B: an outsourcing facility, defined in the statute as a facility at one location engaged in compounding sterile drugs that has registered with FDA. It is not required to be a licensed pharmacy, and a licensed pharmacist must supervise the compounding.

Prescriptions

  • 503A: the drug is compounded for an identified individual patient on receipt of a valid prescription. The statute allows limited quantities in advance where the pharmacist or physician has a history of receiving valid prescriptions for the drug.
  • 503B: the statute says an outsourcing facility may or may not obtain prescriptions for identified individual patients.

Oversight and manufacturing standards

  • 503A: FDA says these pharmacies are not subject to current good manufacturing practice (CGMP) requirements, and that state boards of pharmacy have primary responsibility for day-to-day oversight.
  • 503B: must register with FDA each year, follow CGMP, report adverse events, and is inspected by FDA on a risk-based schedule.

Ingredients and labels

  • 503A: a bulk drug substance must meet a USP or National Formulary monograph, be a component of an FDA-approved drug, or appear on the list FDA develops by regulation. A 503A compounder may not regularly compound products that are essentially copies of a commercially available drug.
  • 503B: a bulk drug substance must be on FDA's 503B bulks list or on the drug shortage list, and must come with a valid certificate of analysis.
  • 503B labels must say that the drug is a compounded drug and carry the facility's name, lot number and other listed details.

What neither one is

  • Compounded drugs are not FDA-approved, and FDA does not verify their safety, effectiveness or quality before they are marketed.
  • Neither category covers a seller labeled “research use only.” That is a different question, which FDA's warning letters address.
  • The statute has further conditions for each category. We summarize the main ones and cite the text for the rest.

Sources

Quotes are from the FDA documents above, retrieved Oct. 5, 2026. FDA can revise those pages. See something wrong? Request a correction.

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