General information about U.S. regulatory definitions and published research, not legal or medical advice.

"Custom peptide" names two different things that sit under two different bodies of law. One is a made-to-order synthetic sequence built in a lab. The other is a patient-specific compounded preparation, which under 21 U.S.C. § 353a(a) escapes FDA premarket approval only when it is compounded for an identified individual patient on receipt of a valid prescription. Conflating the two is the most common — and most expensive — mistake in this market. Below is a primary-sourced map of both, plus what the published data says about what actually arrives in the vial.

What is a custom peptide?

A custom peptide is a peptide built to a sequence you specify rather than pulled from a catalog, and the term splits along a legal seam. In the laboratory sense, it is a synthetic sequence assembled residue by residue — typically by solid-phase peptide synthesis (SPPS), with purity assessed by HPLC and identity confirmed by MALDI-ToF mass spectrometry, per a 2025 method paper in *J Biomed Mater Res A*. In the pharmacy sense, it is a compounded preparation, and 21 U.S.C. § 353a(a) exempts such a product from FD&C Act §§ 501(a)(2)(B), 502(f)(1), and 505 — CGMP, adequate-directions labeling, and new-drug approval — only when it is made for an identified individual patient on a valid prescription.

Neither sense means "approved." Approved peptide drugs are a separate, much smaller set: as of 2020, over 70 therapeutic peptide products had been approved as new chemical entities in the United States, Europe, and Japan, with more than 100 commercial products available globally. Those come with FDA-reviewed labels — semaglutide's OZEMPIC label describes a GLP-1 analogue with 94% sequence homology to human GLP-1, molecular formula C187H291N45O59, MW 4113.58 g/mol, and carries a boxed warning for thyroid C-cell tumors in rodents. A custom peptide has none of that dossier behind it.

Can a compounding pharmacy legally make a custom peptide?

Only if the bulk peptide clears one of three gates — and today, most cannot. Under 21 U.S.C. § 353a(b)(1)(A)(i), a bulk drug substance may be used in § 503A compounding only if it (I) complies with an applicable USP or NF monograph and the USP chapter on pharmacy compounding, (II) is a component of an FDA-approved drug, or (III) appears on the FDA-issued 503A bulks list. That list lives at 21 C.F.R. § 216.23 and contains exactly six substances — Brilliant Blue G, cantharidin (topical only), diphenylcyclopropenone (topical only), N-acetyl-D-glucosamine (topical only), squaric acid dibutyl ester (topical only), and thymol iodide (topical only). No peptide is on it. The final rule establishing that list took effect March 21, 2019, and declined to include four other nominated substances; FDA evaluates nominations on four criteria — physical and chemical characterization, safety issues, evidence of effectiveness or the lack of it, and historical use in compounded drug products.

Two more § 503A constraints bind the supply chain. The bulk substance must be manufactured by an FDA-registered establishment and arrive with a valid certificate of analysis (§ 353a(b)(1)(A)(ii)–(iii)), and a compounder may not "compound regularly or in inordinate amounts... any drug products that are essentially copies of a commercially available drug product" (§ 353a(b)(1)(D)). The 503B outsourcing-facility path differs but is not looser: under 21 U.S.C. § 353b(a), a 503B facility may compound without a patient-specific prescription, but its exemptions do not include § 501(a)(2)(B) — CGMP still applies — and it may use a bulk substance only if that substance is on FDA's clinical-need list or the drug is on the shortage list. On May 1, 2026, FDA proposed **not** to place semaglutide, tirzepatide, or liraglutide on the clinical-need list, stating that it "does not interpret supply issues, such as backorders, to be within the meaning of 'clinical need'"; comments closed June 30, 2026.

Why is 40 amino acids the dividing line?

Because that number decides whether your molecule is a drug or a biologic — and biologics cannot be compounded. FDA defines a protein as "any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size" at 21 C.F.R. § 600.3(h)(6); where two or more chains associate as they do in nature, size is the total across chains. "Protein" is an enumerated category in the statutory definition of "biological product," and no biological product may enter interstate commerce without an effective biologics license, per 42 U.S.C. § 262(i)(1) and § 262(a)(1).

The date that hard-wired this was March 23, 2020. FDA's final rule adopting the >40-residue protein definition took effect that day, the same day the BPCI Act's 10-year transition period ended — after which, per FDA's guidance notice at 83 FR No. 238, an approved § 505 application for a biological product "shall be deemed to be a license... under section 351 of the PHS Act," and FDA will approve no further § 505 applications for products subject to the transition. That is the mechanism behind a question people ask constantly: an anti-obesity polypeptide under 40 amino acids "is no longer considered a 'biological product' by the FDA," which is why semaglutide and tirzepatide were compoundable as drugs while insulin and human growth hormone were not.

Which peptides is FDA actually deciding on right now?

Seven, as of last week. At its July 23–24, 2026 meeting, FDA's Pharmacy Compounding Advisory Committee took up seven peptide bulk drug substances — free base and acetate forms of each — for the 503A bulks list: BPC-157 (ulcerative colitis), KPV (wound healing and inflammatory conditions), TB-500 (wound healing), MOTs-C (obesity and osteoporosis), Emideltide / delta sleep-inducing peptide (opioid withdrawal, chronic insomnia, narcolepsy), Semax (cerebral ischemia, migraine, trigeminal neuralgia), and Epitalon (insomnia), per the meeting notice at 91 FR No. 73. The same committee had considered AOD-9604, CJC-1295 (including DAC and trifluoroacetate forms), and thymosin alpha-1 at its December 4, 2024 meeting. Committee agendas are verified here; the votes are not — read the docket for outcomes, not this page.

Do not read a hearing as a green light. FDA classified BPC-157 as a Category 2 bulk drug in September 2023 over "concerns about safety, impurities, and insufficient human data"; only three human studies of it have been published, and WADA banned it in 2022 under the S0 Unapproved Substances category, per a 2025 review in *Curr Rev Musculoskelet Med*. A 2026 structured review in *JBJS Rev* reaches the same conclusion more broadly: injectable peptides including BPC-157, TB-500, ipamorelin, and CJC-1295 remain "investigational, with uncertain safety profiles, product quality concerns, and widespread antidoping restrictions," and clinical adoption "has outpaced high-quality evidence and regulatory consensus."

How is a custom peptide actually made — and what limits the quality?

Chemistry sets a ceiling long before marketing does. In SPPS, crude purity before purification is typically well under 100%: one 2025 method paper reports 70% average crude purity for optimized manual synthesis versus 50% for microwave-assisted automated synthesis. Length compounds the problem — "difficult sequences" rich in hydrophobic side chains aggregate on-resin, and above roughly 50–60 amino acids they "remain a challenge even when applying automated peptide synthesis protocols," because poor solvation blocks complete deprotection and coupling.

Then there is what comes along for the ride. Fmoc-based SPPS "provides final products with significant amounts of trifluoroacetate ions"; TFA⁻ can inhibit cell proliferation and alter peptide conformation, and in one model peptide the lyophilizate contained 189.94 µg of TFA⁻ per 1 mg, per a 2017 counter-ion exchange study in *Int J Pept Res Ther*. None of that is visible from a product page. It is visible from an HPLC trace and a mass spec — which is why documentation, not adjectives, is the only thing worth comparing. Browse the Cornerstone Peptide catalog, where each item is listed with its identity and analytical documentation so you can check it against the methods above.

How risky are custom peptides bought online?

The measured answer is worse than most buyers assume. A 2024 study in the *Journal of Medical Internet Research* identified 59 unique illegal online pharmacies selling semaglutide without a prescription. Of six test purchases, three vials arrived and three prefilled-pen orders were outright non-delivery scams. The delivered vials contained 28.56%–38.69% more semaglutide than labeled, had measured purity of 7.7%–14.37% against a claimed 99%, and contained endotoxin in every sample (2.1645–8.9511 EU/mg). All were classified as substandard, falsified, and counterfeit. A 2026 critical review in *J Sports Med Phys Fitness* describes the same conditions structurally: the peptide supply chain is "largely unregulated" and "products are often mislabeled or contaminated."

The statutory backdrop is unambiguous even where enforcement practice is not. A drug is misbranded if its labeling is "false or misleading in any particular," or unless its labeling bears "adequate directions for use" and adequate warnings against dangerous use, under 21 U.S.C. § 352(a) and (f). And 21 U.S.C. § 355(a) provides that "no person shall introduce or deliver for introduction into interstate commerce any new drug, unless an approval of an application filed pursuant to subsection (b) or (j) is effective with respect to such drug."

Frequently asked questions

What is a custom peptide? Two things: a made-to-order synthetic sequence produced by solid-phase peptide synthesis for laboratory use, and a patient-specific compounded preparation, exempt from FDA premarket approval only when made for an identified individual patient on a valid prescription under 21 U.S.C. § 353a(a).

Is a custom peptide FDA approved? No. Compounded drugs are *exempt from* FD&C Act § 505 approval rather than approved under it, and a new drug otherwise may not enter interstate commerce without an effective approved application (21 U.S.C. § 355(a)).

Can a pharmacy compound a custom peptide for me? Only if the bulk substance meets a USP/NF monograph, is a component of an approved drug, or is on the 503A bulks list — and that list at 21 C.F.R. § 216.23 holds six substances, none of them a peptide.

Why does 40 amino acids matter? Anything greater than 40 residues with a defined sequence is a "protein" under 21 C.F.R. § 600.3(h)(6) and therefore a biological product requiring a license under 42 U.S.C. § 262 — outside the compounding pathway entirely.

What did testing find in online-sold peptides? In a 2024 *JMIR* analysis, delivered semaglutide vials ran 28.56%–38.69% over labeled content, measured 7.7%–14.37% purity against a claimed 99%, and carried detectable endotoxin in every sample.