What peptides are, how they work, and what the data really shows.

What is a peptide?

A peptide is a short chain of amino acids — typically between 2 and 50 — held together by peptide bonds. They sit between single amino acids and full proteins on the size scale, and the human body uses thousands of them as signaling molecules: hormones (insulin, oxytocin), growth factors, neuromodulators, and immune messengers.

Therapeutic peptides vs. proteins

Therapeutic peptides offer the receptor specificity of biologics with the manufacturing simplicity of small molecules. Most have short plasma half-lives (minutes to hours), high target selectivity, and predictable degradation into amino acids — which is why they tend to have favorable safety profiles in research settings.

Why compounding matters

Most of the 12 reclassified peptides are not FDA-approved drugs. They exist in a regulatory space called 503A compounding — pharmacies that prepare individualized prescriptions for specific patients. When the FDA placed them on Category 2 in 2023, it effectively banned that pathway. The April 2026 reclassification reopens it, pending PCAC review.

The research landscape

• BPC-157 — 100+ peer-reviewed animal studies; limited human data
• TB-500 / Thymosin Beta-4 — Phase 2 trials in dry eye (RGN-259)
• Semax — Approved in Russia for ischemic stroke recovery
• Epitalon — 12-year Russian longevity cohort study
• Melanotan II — Direct precursor to FDA-approved bremelanotide (Vyleesi)
• MOTS-c, KPV, DSIP, GHK-Cu, LL-37, DiHexa, PEG-MGF — Substantial preclinical literature; varying levels of human data

Honest about the gaps

Critics — including FDA career scientists and groups like the AMA — have noted that several of these peptides lack the large-scale human trials normally expected for compounding approval. Tuesdays takes that seriously: every member is on a monitored protocol with quarterly labs, and we publish post-market safety data.