FDA Category 1 vs Category 2 Peptides

Peptide Compounding Regulation

19 Peptides Classified Category 2

Between 2023 and 2024, the FDA placed 19 peptides on the Category 2 bulk drug substances list, banning their use by compounding pharmacies and reshaping the entire peptide therapy landscape.

FDA Certain Bulk Drug Substances List, 2024

FDA Certain Bulk Drug Substances List, 2024

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The distinction between Category 1 and Category 2 on the FDA's bulk drug substances list determines whether a peptide can be legally compounded by licensed pharmacies in the United States. Category 1 means the substance can be used while under evaluation. Category 2 means the FDA has identified "significant safety risks" and will not permit compounding. When the FDA placed 19 peptides into Category 2 between 2023 and 2024, it did not just reclassify chemicals on a government list. It cut off patient access to peptides that thousands of clinicians had been prescribing through compounding pharmacies, while the same molecules remained available through unregulated online vendors. For the full legal framework behind this system, see our pillar article on the 503A bulk drug substances list.

What Category 1 and Category 2 actually mean

The category system was created as part of the FDA's interim policy for evaluating bulk drug substances nominated for inclusion on the 503A list. It was always intended as a temporary framework, not a permanent regulatory structure.

Category 1 includes substances that are under evaluation with adequate supporting safety information and no significant safety risks identified at the time of review. Compounding pharmacies can use Category 1 substances while the FDA completes its evaluation, provided all other legal requirements are met (valid prescription, licensed pharmacist, individual patient). The FDA stated it "does not intend to take action against a compounder" using Category 1 substances.

Category 2 includes substances that the FDA determined present "significant safety risks" based on the information submitted with their nominations. Compounding with Category 2 substances is prohibited under the interim policy. The FDA's position is unambiguous: Category 2 means "do not compound."

Category 3 (now retired) included substances with insufficient supporting information for evaluation. These could not be compounded either, but the barrier was informational rather than safety-based. More data might have moved them to Category 1.

The distinction matters because it reversed the default assumption. Before the category system, a peptide could generally be compounded if a pharmacist had the raw material and a valid prescription. After Category 2 classification, the burden shifted: the substance was restricted until the FDA affirmatively cleared it.

The 19 peptides that landed in Category 2

Between October 2023 and December 2024, the FDA placed the following substances on the Category 2 list under the 503A interim policy:

503A Category 2 peptides:

• BPC-157 (Body Protection Compound-157)
• Cathelicidin LL-37
• Dihexa acetate
• Emideltide (DSIP / Delta Sleep-Inducing Peptide)
• Epitalon
• GHK-Cu (injectable formulation)
• Kisspeptin-10
• KPV (alpha-MSH fragment)
• Mechano Growth Factor, Pegylated (PEG-MGF)
• Melanotan II
• MOTs-C (mitochondrial-derived peptide)
• Semax (heptapeptide)
• Thymosin beta-4 fragment (LKKTETQ / TB-500)

503B Category 2 peptides (outsourcing facilities):

• GHRP-2 (Growth Hormone Releasing Peptide-2)
• GHRP-6 (Growth Hormone Releasing Peptide-6)
• Ibutamoren mesylate (MK-677, technically not a peptide but a peptide mimetic)
• Ipamorelin acetate

Several non-peptide substances also appeared on Category 2 (cesium chloride, domperidone, germanium sesquioxide, quinacrine hydrochloride), but the peptide restrictions drew the most public attention because they affected a rapidly growing area of clinical practice.

For a detailed look at how the classification specifically affected BPC-157, see BPC-157 and the FDA: The Category 2 Classification Explained. For the complete timeline of regulatory actions, see The Timeline of FDA Peptide Restrictions.

The FDA's stated safety concerns

The FDA did not apply Category 2 arbitrarily. Each classification came with specific safety rationales documented in the agency's published list. The concerns fall into several categories.

Immunogenicity risk. Peptides, particularly when injected, can trigger immune responses. The body may produce antibodies against the peptide itself or against aggregated forms of the peptide that present novel epitopes to the immune system. Colalto et al. (2024) reviewed the regulatory complexities of peptide quality assessment and noted that peptide-related impurities can exhibit unpredictable immunogenic properties that current analytical methods may not fully characterize.^[1] Roberts et al. (2024) demonstrated this principle with salmon calcitonin, showing that specific peptide impurities could be assessed for immunogenicity risk using computational and in vitro methods.^[2]

Manufacturing impurities. Compounded peptides are synthesized through solid-phase peptide synthesis (SPPS), and the process generates truncated sequences, deletion peptides, racemized amino acids, and other byproducts. Currier et al. (2008) documented that commercial synthetic peptides contain impurities sufficient to alter vaccine trial assessments, establishing that even research-grade peptide purity may not meet clinical safety standards.^[3] Verbeken et al. (2012) showed that impurity profiles can measurably change the biological activity of peptide preparations in functional tissue assays.^[4]

Aggregation. Peptides can form aggregates during storage, particularly at higher concentrations and in certain formulation conditions. Aggregated peptides present different immunogenic profiles than monomeric peptides and can trigger more severe immune reactions. This is a well-established concern in the biopharmaceutical industry but less well-controlled in compounding pharmacy settings.

Absence of human clinical data. For peptides like dihexa, KPV, MOTs-C, and PEG-MGF, the FDA noted that no human exposure data existed at the time of nomination. The agency's position was that injecting substances with zero human clinical data through compounding pharmacies bypasses the safety evaluation that the drug approval process is designed to provide.

Specific organ toxicity. Some Category 2 substances had documented safety signals beyond general peptide concerns. Melanotan II was associated with melanoma risk and priapism. Ibutamoren was linked to congestive heart failure potential. These substance-specific risks provided additional justification beyond the class-level peptide safety arguments.

The September 2024 removals

On September 20, 2024, the FDA removed five substances from the 503A Category 2 list: AOD-9604, CJC-1295, ipamorelin acetate, thymosin alpha-1, and selank acetate. The removal was based on the nominators withdrawing their nominations, not on the FDA reversing its safety determination.

This distinction matters. The FDA did not say these peptides were safe. It said the nominations that placed them under evaluation were withdrawn, so the regulatory basis for their Category 2 listing no longer existed. The peptides were referred to the Pharmacy Compounding Advisory Committee (PCAC) for further review, which is a necessary step before potential Category 1 designation.

The removal from Category 2 did not immediately restore compounding access. These peptides entered a regulatory gray zone: no longer explicitly restricted, but not yet authorized for compounding either. PCAC review dates were scheduled, and the peptides' ultimate status would depend on the committee's evaluation of safety and clinical utility data.

What the impurity research shows

The FDA's immunogenicity and impurity concerns are grounded in published research, not regulatory speculation.

De et al. (2025) reviewed current methods for assessing immunogenicity of generic peptide impurities and found that standard approaches may be insufficient. The paper described how even minor structural variants of a peptide, such as truncated sequences or deamidated forms, can bind to immune cell receptors differently than the parent molecule, potentially triggering antibody responses that cross-react with endogenous proteins.^[5]

Tang et al. (2025) examined innate immune response modulating impurities (IIRMIs) in synthetic liraglutide and found detectable levels of impurities capable of activating innate immune pathways.^[6] This is significant because liraglutide is an FDA-approved drug manufactured under CGMP conditions. If IIRMI contamination occurs in FDA-approved manufacturing, the risk in less controlled compounding environments is presumably higher.

Hach et al. (2024) directly compared the properties of compounded GLP-1 polypeptide drugs to their commercially manufactured counterparts and found measurable differences in quality attributes related to the manufacturing process and compounding conditions.^[7] The study established that compounding is not a neutral process; the same molecule can emerge from a compounding pharmacy with different physical and chemical properties than the FDA-approved version.

The January 2025 policy change

On January 7, 2025, the FDA finalized its interim policy on bulk drug substances, making several changes that directly affected the category system.

The agency stopped assigning new Category 1 designations. Substances nominated after this date would no longer receive interim categories. Instead, they would enter a full evaluation process, and compounding would not be permitted until the review was complete. This process can take years.

The change effectively closed the door that the interim category system had kept open. Previously, a newly nominated substance could receive a Category 1 designation and be compounded immediately while evaluation continued. Under the finalized policy, the default position shifted to restriction until proven otherwise.

For existing Category 1 substances, the policy maintained their status. Compounding pharmacies could continue using substances already listed in Category 1. Category 2 restrictions remained in place.

The February 2026 announcement

On February 27, 2026, HHS Secretary Robert F. Kennedy Jr. announced during a public appearance that approximately 14 of the 19 peptides previously placed on Category 2 would be moved back to Category 1, restoring their availability through licensed compounding pharmacies.

The announcement represented a significant policy reversal from the previous administration's regulatory direction. However, as of March 2026, no formal reclassification through the FDA regulatory process had been published. The FDA had not updated its Category 2 list, and no Federal Register notice had been issued.

This gap between announcement and implementation is consequential. Until the FDA formally modifies the Category 2 list, compounding pharmacies that produce these peptides are technically in violation of the interim policy. Some compounding pharmacies resumed production based on the announcement; others waited for formal regulatory action.

Approximately 5 peptides (reportedly including melanotan II and certain growth hormone secretagogues) were expected to remain on Category 2 even under the proposed reclassification, though the specific list had not been published.

The paradox of the gray market

The Category 2 classification created an unintended consequence that critics have consistently highlighted. Before Category 2, patients could obtain peptides like BPC-157 and thymosin beta-4 through licensed compounding pharmacies operating under physician oversight, with some level of quality control and dose standardization.

After Category 2, the same molecules remained available through online vendors selling them labeled "for research use only." These vendors operate outside FDA jurisdiction for compounding, often ship internationally, and have no obligation to test for purity, sterility, or endotoxin levels. The regulatory action designed to protect patients from unsafe peptides may have redirected some patients toward less safe sources. For more on the legal fiction of gray-market peptides, see our dedicated article.

A 2026 pharmacovigilance study by McCall et al. analyzed adverse event reports for compounded GLP-1 receptor agonists through the FDA's FAERS database, providing the first systematic safety data on compounded peptide formulations.^[8] Mayfield et al. (2026) published a primer for physicians on injectable peptide therapy, noting the clinical reality that patients frequently access peptides through unregulated channels when compounding access is restricted.^[9]

This dynamic does not invalidate the FDA's safety concerns. Immunogenicity, impurity, and aggregation risks are real regardless of the source. But it does raise the question of whether Category 2 classification achieved its intended public health goal, or whether it shifted risk from a partially regulated setting to a fully unregulated one.

How the regulation framework may evolve

For a complete overview of all FDA regulatory mechanisms affecting peptides, see How the FDA Regulates Peptides.

The trajectory suggests the rigid Category 1/Category 2 binary is unlikely to survive in its current form. The January 2025 policy change already dismantled the interim category system for new nominations. The February 2026 announcement signaled political will to reverse many Category 2 designations. And the PCAC review process for the five removed peptides may produce a template for evidence-based evaluation that replaces blanket classification.

What remains unclear is whether any replacement framework will address the underlying tension: the FDA's mandate to ensure drug safety versus the clinical reality that many peptides have been used for years without formal FDA evaluation. The gap between these two positions is where the Category 1 vs Category 2 debate lives, and it is not likely to close soon.

The Bottom Line

The FDA's Category 1 and Category 2 classification system for bulk drug substances determined which peptides could legally be compounded by licensed pharmacies. Nineteen peptides were placed on Category 2 between 2023 and 2024, citing immunogenicity risks, manufacturing impurities, aggregation potential, and absence of human clinical data. Five were removed in September 2024 when their nominations were withdrawn. In February 2026, the HHS Secretary announced plans to reclassify approximately 14 of the 19 back to Category 1, though formal FDA action had not been completed. The underlying safety concerns about peptide impurities and immunogenicity are supported by published research, but the Category 2 classification also pushed some patients toward less regulated sources.

Frequently Asked Questions

What is the difference between Category 1 and Category 2 peptides?

Category 1 peptides can be used by compounding pharmacies while the FDA evaluates them; the FDA will not take enforcement action against compounders using Category 1 substances. Category 2 peptides have been identified by the FDA as presenting "significant safety risks" and cannot be compounded. The distinction determines whether a licensed pharmacist can legally prepare that peptide for a patient with a valid prescription.

Which peptides are on the FDA Category 2 list?

As of early 2026, the 503A Category 2 list includes BPC-157, cathelicidin LL-37, dihexa, emideltide (DSIP), epitalon, GHK-Cu (injectable), kisspeptin-10, KPV, melanotan II, MOTs-C, PEG-MGF, semax, and thymosin beta-4 fragment. The 503B Category 2 list adds GHRP-2, GHRP-6, ibutamoren, and ipamorelin. Five peptides (AOD-9604, CJC-1295, ipamorelin, thymosin alpha-1, selank) were removed from Category 2 in September 2024.

Are peptides becoming legal again in 2026?

In February 2026, HHS Secretary Robert F. Kennedy Jr. announced that approximately 14 of the 19 restricted peptides would be moved back to Category 1, which would restore compounding access. However, no formal FDA reclassification had been published by March 2026. Until the FDA officially updates the Category 2 list, the legal status of these peptides for compounding remains technically restricted.

Why did the FDA ban peptides from compounding?

The FDA cited several safety concerns: immunogenicity risk (the potential for injected peptides to trigger harmful immune responses), manufacturing impurities from solid-phase synthesis, peptide aggregation during storage, and the absence of human clinical trial data for many nominated substances. Research has shown that synthetic peptide impurities can alter biological activity and trigger immune responses that differ from the intended therapeutic effect.

Can compounding pharmacies still make sermorelin?

Yes. Sermorelin maintained Category 1 status throughout the Category 2 restrictions. It can be compounded under 503A rules with a valid prescription. Other peptides that remained available for compounding include gonadorelin and NAD+. Peptides with USP monographs or that are components of FDA-approved drugs can also be compounded regardless of the bulks list.

What is the PCAC and what does it do for peptides?

The Pharmacy Compounding Advisory Committee (PCAC) is an FDA advisory committee that evaluates bulk drug substances nominated for inclusion on the 503A or 503B compounding lists. When peptides are removed from Category 2 or newly nominated, the PCAC reviews safety data, clinical utility, and public health implications before recommending whether the substance should be permitted for compounding. PCAC review is a necessary step toward Category 1 designation.