MOTS-c side effects: what is known about safety

What the regulator looked for and did not find

FDA’s evaluation searched the published literature, DailyMed, Drugs@FDA, the Cochrane Database and its own adverse event reporting system. On safety it concluded that there is a lack of clinical and nonclinical safety information on the use of MOTS-c, that it was particularly concerned about the absence of human data by any route of administration, and that MOTS-c is a peptide for which there is a lack of information to assess its immunogenic safety risk [FDA 2026].

Its adverse-event searches, run through February 2024 and again through March 2025, retrieved no reports. The briefing document is explicit that this is weak reassurance: compounders operating under section 503A generally do not report adverse events to the agency, so FDA may simply not know about them [FDA 2026].

The specific concerns FDA raised

• Immunogenicity. Peptides can provoke immune responses, and that risk is higher with injected routes. Because no published data describe the impurities or aggregates present in MOTS-c material, FDA wrote that it could not rule out immunogenicity associated with them [FDA 2026].
• Characterisation. Both the free base and the acetate were judged not well characterised, citing naming that does not follow established chemical nomenclature standards and missing impurity, aggregate and endotoxin data in the public literature [FDA 2026].
• Unknown target organs. With the molecular target unidentified, the agency wrote that it is difficult to predict which organs might be targeted and affected [FDA 2026]. That makes it hard to know what to monitor, even in principle.
• No dose-response data. Assessments of dose-response relationships are missing, so there is no basis for distinguishing a quantity with no effect from one with a harmful effect [FDA 2026].

What the registered trials will add

The Phase 2a trial in prediabetes lists the incidence of treatment-emergent adverse events as a co-primary outcome alongside its insulin-sensitivity endpoint [NCT07505745]. That is the first systematic human safety data on administered MOTS-c, and it is not expected to complete until 2028.

The earlier Phase 1a/1b programme for CB4211, a MOTS-c analogue, listed adverse events, laboratory evaluations, vital signs, ECG parameters, physical examinations and injection-site assessments as its primary outcomes. It completed in 2021 and posted no results to the registry [NCT03998514]. An analogue is a different molecule, so even published results would not transfer directly.

Product quality as a separate risk

Material sold online is outside any approved supply chain. FDA found no applicable USP or NF monograph for either form and no certificate of analysis in the nomination package establishing identity and purity [FDA 2026]. That means purity, identity and endotoxin content vary by supplier and batch, independently of anything about the peptide itself.

Other considerations

USADA’s guidance notes that MOTS-c is not approved by FDA for human use or compounded medications [USADA], and it is prohibited in competitive sport at all times. See regulatory status for the full picture, and dosage for why no amount has been established.