Compounding: 503A vs 503B
US pharmacy compounding is regulated under sections 503A and 503B of the Federal Food, Drug, and Cosmetic Act. This guide explains each pathway, the bulk drug substances lists, FDA's interim categories, the PCAC's role and how compounding differs from research-use-only sales.
What compounding is
Compounding is the preparation of a medication by combining, mixing or altering ingredients to meet the needs of a patient, for example when an approved product is unavailable or a different formulation is required. Compounded drugs are not FDA-approved: they do not go through premarket review of safety, effectiveness or manufacturing quality, although they are subject to federal and state requirements.
Federal law recognizes two main pathways, described in sections 503A and 503B of the Federal Food, Drug, and Cosmetic Act. Section 503B was created by the Drug Quality and Security Act of 2013, which followed a fungal meningitis outbreak linked to contaminated compounded injections. The two pathways differ in who may compound, under what conditions and with which ingredients. State pharmacy boards also regulate compounding, and their rules vary.
Section 503A: traditional pharmacy compounding
Section 503A covers compounding by a licensed pharmacist in a state-licensed pharmacy or by a licensed physician. Its central condition is that drugs are compounded based on a valid prescription for an identified individual patient, although limited anticipatory compounding based on a history of prescriptions is permitted. Pharmacies operating under 503A are overseen primarily by state boards of pharmacy and are exempt from certain federal requirements, including current good manufacturing practice and drug approval, if they meet the section's conditions.
Those conditions include restrictions on the ingredients that may be used, limits on compounding drugs that are essentially copies of commercially available approved products, and limits on distribution across state lines. Ingredients must meet applicable compendial standards, such as those of the United States Pharmacopeia, where they exist.
Section 503B: outsourcing facilities
Section 503B created a category called outsourcing facilities. These are compounders that register voluntarily with FDA, are subject to FDA inspection on a risk-based schedule, must comply with current good manufacturing practice requirements and must report the products they compound to FDA. In exchange, they may compound drugs with or without patient-specific prescriptions and can supply hospitals, clinics and practitioners for office use.
Outsourcing facilities face their own restrictions. They may not compound drugs that are essentially copies of approved drugs, they must label products as compounded drugs, and they are limited in which bulk drug substances they can use. Because they produce larger volumes under federal oversight, 503B facilities are held to quality standards closer to those of drug manufacturers than 503A pharmacies are.
- 503A: state-licensed pharmacy or physician; patient-specific prescriptions; primarily state oversight; not subject to cGMP.
- 503B: FDA-registered outsourcing facility; may compound without patient-specific prescriptions; FDA inspection; cGMP required.
Bulk drug substances lists
A bulk drug substance is an active ingredient used to make a finished drug. Each pathway restricts which bulk substances may be used. Under 503A, a bulk substance must comply with an applicable USP or National Formulary monograph, be a component of an FDA-approved drug, or appear on the list of bulk drug substances that may be used in 503A compounding, commonly called the 503A bulks list. Under 503B, a bulk substance must appear on the 503B bulks list, for which FDA has determined there is a clinical need, or be used to compound a drug that appears on FDA's drug shortage list.
Substances are added to these lists through nomination, evaluation by FDA, consultation with an advisory committee for the 503A list, and rulemaking. The process is slow, and many nominated substances remain under evaluation for years. Peptides raise particular difficulties because identity, characterization, salt form and impurity profiles may not be well defined for substances that have never been through drug approval.
FDA's interim categories and the PCAC
While nominations are evaluated, FDA has published interim policies that sort nominated 503A substances into categories. Category 1 covers substances under evaluation that may be eligible, which FDA generally does not intend to act against in compounding while evaluation continues, provided other conditions are met. Category 2 covers substances under evaluation for which FDA has identified significant safety concerns, which compounders are not expected to use. Category 3 covers substances nominated without adequate supporting information. Categories can change, and they are policy statements rather than final rules.
The Pharmacy Compounding Advisory Committee (PCAC) is an FDA advisory committee that reviews nominated substances and votes on whether they should be added to the 503A bulks list. Its votes are advisory; FDA decides through notice-and-comment rulemaking. In July 2026, the PCAC voted to recommend adding BPC-157, KPV, TB-500, MOTS-c, epitalon and semax to the 503A list and did not support emideltide (DSIP), while FDA's own reviewers had recommended against. No final FDA action has been announced; see the regulatory tracker for current status.
Approved drugs, shortages and limits
Compounding generally may not be used to produce copies of approved drugs that are commercially available. During shortages listed by FDA, compounders have had additional flexibility, and the end of a shortage changes what may lawfully be compounded. This was prominent for semaglutide and tirzepatide, whose shortage-list status affected compounding during their periods of high demand.
Some substances cannot be compounded at all. Ingredients of drugs withdrawn for safety reasons and substances on FDA's list of drugs that are demonstrably difficult to compound are excluded. FDA has stated that investigational compounds such as retatrutide, which are not approved and not on a bulks list, cannot be used in compounding.
How compounding differs from research-use-only sales
Research-use-only (RUO) materials are sold for laboratory research and are labeled as not for human or veterinary use. They are not drugs in the regulatory sense of having been prescribed, compounded or approved, and their sale is not a compounding activity. A peptide's status on a bulks list, or a PCAC vote, does not change the status of RUO material, which remains outside the drug supply chain.
The distinction has practical consequences. Compounded drugs are prepared by licensed professionals for patients, under state and sometimes federal oversight, from bulk substances that must meet specific requirements. RUO materials carry no such framework, and their quality depends on the supplier and on testing such as that described in how to read a COA. This wiki covers peptides in a research context and does not provide guidance on obtaining or using compounded medications.
Frequently asked questions
What is the main difference between 503A and 503B?
A 503A pharmacy compounds based on prescriptions for individual patients and is overseen mainly by state boards of pharmacy without cGMP requirements. A 503B outsourcing facility registers with FDA, must follow cGMP, is inspected by FDA and may compound without patient-specific prescriptions for supply to healthcare facilities. Each pathway has its own bulk substances rules.
Does a PCAC vote make a peptide legal to compound?
No. The Pharmacy Compounding Advisory Committee gives advice; FDA decides whether to add a substance to the 503A bulks list through notice-and-comment rulemaking. Until FDA acts, a favorable vote does not change a substance's status. Interim category placements also matter and can change, so current FDA statements should be checked.
Is a research-use-only peptide the same as a compounded peptide?
No. Compounded drugs are prepared by licensed pharmacists or physicians, or registered outsourcing facilities, for patients under federal and state rules on ingredients and quality. Research-use-only materials are sold for laboratory research, labeled not for human use, and sit outside the compounding framework entirely, regardless of a peptide's bulks-list status.
Compounds mentioned
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