Peptide side effects

What are the side effects of licensed peptide drugs?

The side effects of a licensed drug are collected in pre-approval clinical trials and through post-approval reports and are written in the product information (SmPC in the EU; in Türkiye, the Turkish KÜB) in frequency classes such as “very common” (more than 1 in 10 people), “common” (1–10 in 100), and “uncommon.” The summaries below are at class level; the details of each drug are in its own guide.

GLP-1 drugs (semaglutide, liraglutide, tirzepatide)

According to the EMA’s Ozempic product information, with semaglutide 0.5 mg and 1 mg, nausea occurred in 17% and 19.9% of patients, diarrhea in 12.2% and 13.3%, and vomiting in 6.4% and 8.4%, respectively; most events were mild to moderate in severity and short-lived, and the discontinuation rate was 3.9% and 5%. ^[1] The same product information contains the following warnings: ^[1]

• Acute pancreatitis: Has been observed with GLP-1 receptor agonists; if suspected, the drug is stopped, and if confirmed, it is not restarted. In phase 3a trials, confirmed acute pancreatitis was 0.3% with semaglutide and 0.2% with comparators; in the two-year cardiovascular trial it was 0.5% versus 0.6% with placebo.
• Gallstones (cholelithiasis): Included in the side effect table.
• Diabetic retinopathy complications: In the two-year trial of 3,297 people with type 2 diabetes at high cardiovascular risk, these occurred in 3% in the semaglutide group and 1.8% in the placebo group; the difference was observed in patients with known diabetic retinopathy who were using insulin.
• Hypoglycemia: Predominantly when used together with a sulfonylurea or insulin.
• Fluid loss: Fluid loss due to nausea, vomiting, and diarrhea may in rare cases lead to worsening kidney function.

The thyroid warning is written differently by two authorities. According to the EMA, non-lethal thyroid C-cell tumors seen in rodents are a class effect of GLP-1 receptor agonists; their relevance to humans is considered low but cannot be completely excluded. ^[1] The US label, on the other hand, gives this as a boxed warning: it is unknown whether the drug causes C-cell tumors, including medullary thyroid carcinoma (MTC), in humans, and the drug is contraindicated in people with a personal or family history of MTC or with multiple endocrine neoplasia syndrome type 2 (MEN 2). ^[2] Topics such as optic nerve damage (NAION), aspiration during anesthesia, muscle loss, and weight regain after stopping the drug are covered in the Ozempic side effects, Mounjaro side effects, and Saxenda (liraglutide) side effects guides.

Insulin

According to the EMA’s insulin aspart product information, side effects mostly arise from the pharmacological effect of insulin, and the most commonly reported side effect is hypoglycemia; it is in the “very common” class in the table, and its frequency varies with the patient group, dose regimen, and blood sugar control. At the start of treatment, refraction disorders, edema, and injection site reactions may occur; rapid improvement in blood sugar may be associated with temporary worsening of diabetic retinopathy. The product information also notes the risk of lipodystrophy and cutaneous amyloidosis with repeated injections in the same area. ^[4]

Growth hormone and growth hormone secretagogues

The growth hormone secretagogues covered on this site (CJC-1295, ipamorelin) are not licensed in any country; growth hormone itself (somatropin) is a licensed drug, and its product information describes the known effects of this axis. According to the somatropin product information, in patients with growth hormone deficiency, the extracellular fluid deficit is rapidly corrected at the start of treatment; effects due to fluid retention such as peripheral edema and joint pain are very common, and musculoskeletal stiffness, muscle pain, and numbness-tingling are common; these effects usually appear in the first months and resolve spontaneously or when the dose is reduced. The side effect table also includes carpal tunnel syndrome and type 2 diabetes in adults; somatropin may reduce insulin sensitivity, and people with diabetes or glucose intolerance are closely monitored. ^[3]

How often secretagogues such as CJC-1295 and ipamorelin cause the same effects has not been sufficiently studied in humans; somatropin data cannot be transferred directly to these substances. For ibutamoren (MK-677), from the same class (growth hormone secretagogue) but not a peptide, the FDA writes that a placebo-controlled trial in patients recovering from hip fracture was terminated early because of a heart failure safety signal. ^[5] Details are in the CJC-1295 and Ipamorelin guides.

What are the risks of unlicensed peptides?

For unlicensed “research peptides,” the risk has three layers: the substance’s own unknown effects in humans, the quality of the product (content, purity, sterility), and the absence of a system that collects side effects.

Immune response (immunogenicity)

The FDA gives the same rationale for many peptides nominated for compounding drugs in pharmacies: the risk of immune response by some routes of administration due to aggregation and peptide-related impurities. This rationale appears on the TB-500, ipamorelin, and AOD-9604 lines; the FDA states that it found no human exposure data for products containing TB-500, and for AOD-9604 it identified serious adverse events whose causality is unclear. The FDA also writes that serious adverse events, including death, were reported in a published study in which ipamorelin was given intravenously. ^[5] In its BPC-157 assessment, the FDA states that it cannot exclude the possibility of an immune response because there is no information on impurities and peptide aggregation. ^[6]

Sterility, endotoxin, and purity: what did measured samples show?

Among the critical quality attributes of an injectable product, the FDA lists sterility, bacterial endotoxin testing, and foreign particles. The FDA writes that most of the certificates of analysis it found in the literature for BPC-157 (free base) contained only a purity test result and gave no information on impurity limits or results. ^[6] An FDA warning letter also reminds that injectable products are given directly into the body, sometimes directly into the bloodstream, and therefore bypass some of the body’s basic defenses against toxins and microorganisms. ^[11]

Independent analyses show this gap with numbers:

Two of the authors of the Ashraf study are employees of a data science company, and one is a co-founder of the company. ^[7] The Janvier study was conducted by researchers from the Belgian Scientific Institute of Public Health. ^[8] Although the metal results in the Piatkowski study are reassuring, the deviation in amount shows that a vial with the same label could contain half or nearly twice the dose. ^[9] The instability of peptides to heat and moisture is covered in the How are peptides stored? guide.

Reported cases

Side effect information on unlicensed peptides mostly comes from individual case reports. In the FDA’s BPC-157 assessment, a 40-year-old woman using a product containing BPC-157 and TB-500 labeled “for research use only” subcutaneously for joint and wound healing reported widespread skin darkening and gum darkening after one week; when the product was restarted two weeks later, the same picture recurred, and no new reaction occurred once the product was stopped. The FDA also writes that it is unclear whether reports in the adverse event database can be attributed to BPC-157. ^[6] Details are in the BPC-157 side effects guide.

Counterfeit versions of licensed peptides are also a separate risk group. Zinzi and colleagues found 234 case reports associated with possible counterfeit semaglutide products in the European pharmacovigilance database EudraVigilance between 2018 and 2025; 89.3% of suspected adverse reactions were serious, the most frequently reported were vomiting, nausea, and hypoglycemia, and hypoglycemia was reported disproportionately often with counterfeit products compared with originals. ^[10] This is a reporting analysis; it does not allow calculation of frequency. The risks reported for retatrutide are in the Retatrutide side effects guide.

What does “no reported side effects” mean?

For licensed drugs, side effects do not emerge on their own; they are collected by a system. According to the EU medicines directive, the marketing authorization holder must operate a pharmacovigilance system to fulfill its duties, scientifically evaluate all information, consider options for risk minimization, and take measures when necessary; it audits the system regularly. ^[12] Every side effect line in the product information is the result of this process.

For unlicensed peptides, there is no such marketing authorization holder or system. The party selling the product may be of unclear identity, the product’s content is not verified, and problems experienced by users are not collected systematically. For this reason, online statements such as “side effects unknown” or “no side effects reported” often mean “not monitored”; they are not proof of safety. The FDA’s writing, for many peptides, that “no human exposure data were found” or “there is insufficient information to know whether it could harm humans” describes the same gap. ^[5]

Summary: what is known about which peptide?

Where to go after this guide?

The concept of peptides is in What are peptides?, the legal framework in Are peptides legal?, and the label language of unlicensed products in What is a research peptide?. A new symptom that appears while using a licensed peptide drug is assessed with a physician, together with the warning signs described in the product information and the relevant drug guide. This guide does not recommend personal products, sources, or doses.