Comparative Analysis of Peptide Classes

Introduction to Peptide Classes

Lifters comparing peptides for muscle growth usually look at four groups. GHRPs and GHRH analogs raise the body's own growth hormone (GH), IGF-1 peptides act further down that pathway, and myostatin inhibitors release a separate brake on muscle. Each class includes an approved prescription medicine, but the research-chemical versions have far less human data.

Mechanisms of Action

The primary peptide classes relevant to bodybuilders include:

Growth Hormone Releasing Peptides (GHRPs)

• Examples: GHRP-2, GHRP-6, Hexarelin, Ipamorelin
• Mechanism: Stimulate GH secretion from the anterior pituitary via the ghrelin receptor
• Effects: Raise GH and IGF-1; appetite effects vary (users describe intense hunger with GHRP-6)
• Evidence: Small human studies show GH release, but no trial has measured muscle outcomes. Ipamorelin's evidence is moderate; its one Phase 2 trial missed its primary endpoint.

Growth Hormone Releasing Hormone (GHRH) Analogs

• Examples: Sermorelin, CJC-1295, Tesamorelin
• Mechanism: Mimic the body's own GHRH at the pituitary
• Effects: Pulsatile GH release; users often pair one with a GHRP, a combination not tested in outcome trials
• Approval: As of October 2026, tesamorelin is FDA-approved only for excess abdominal fat in HIV-associated lipodystrophy. CJC-1295 isn't approved anywhere.

Insulin-like Growth Factor-1 (IGF-1) Peptides

• Examples: IGF-1 LR3, DES(1-3) IGF-1, MGF
• Mechanism: Act directly on IGF-1 receptors
• Effects: Promote cell growth and survival in animal and cell studies
• Approval: Mecasermin (Increlex), a recombinant human IGF-1, is prescribed for growth failure in children with severe primary IGF-1 deficiency. Evidence for IGF-1 LR3 is preliminary, with rat data only (see our IGF-1 LR3 profile).

Myostatin Inhibitors

• Examples: Follistatin, ACE-031
• Mechanism: Block myostatin's muscle growth inhibition
• Effects: More lean mass in human antibody trials
• Approval: FDA approved apitegromab (Isembyld), a myostatin-targeting antibody, on September 11, 2026 for spinal muscular atrophy (SMA), with a fracture-risk warning. It's approved for SMA, not muscle building. ACE-031 was dropped in 2013 (see our ACE-031 overview), and research-market follistatin has no human data.

Benefits and Risks Comparison

Legal Implications

As of October 2026, the research-market peptides here aren't approved in the US, and a "research use only" label doesn't protect sellers. FDA's August 2026 warning letters said RUO disclaimers didn't help sellers whose sites made drug claims and sold bacteriostatic water; tesamorelin was among the products named. The approved drugs above are prescription medicines, and research-chemical versions aren't equivalent to them.

WADA's 2026 Prohibited List bans all four classes at all times: GHRPs and GHRH analogues under S2.2.4, IGF-1 and analogues under S2.3, and myostatin inhibitors under S4.3. Our myostatin inhibitors overview covers that group in more detail.

What we still don't know

Nobody has compared these classes head-to-head for muscle gain in healthy adults. It's also unclear whether lean mass preserved by myostatin-pathway antibodies improves strength.

Comparison table