Who shouldn't take peptides: evidence-based contraindications

Most peptide conversations online focus on who should use them. The more important conversation is who shouldn't. A good protocol starts with screening, not with a product.

Active or recent cancer is the single clearest contraindication#

The cancer concern with growth-hormone-axis peptides is specific: they raise insulin-like growth factor-1 (IGF-1). IGF-1 is mitogenic — it promotes cell proliferation. In people with active or recent malignancy, that's a risk signal that doesn't pass a reasonable risk-benefit calculation.

This applies to the whole GH-releasing class: CJC-1295, Ipamorelin, Tesamorelin, Sermorelin, GHRP-2, GHRP-6, Hexarelin, and MK-677 (ibutamoren). Even though a 2020 meta-analysis across 31 studies found no association between IGF-1 levels and cancer incidence in the general population, the population being treated with GH secretagogues is self-selected for bloodwork screening. The meta-analysis doesn't give clearance for people with active cancer.

The blanket rule: personal history of active or recent (within five years) cancer, or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2, is a contraindication for the GH-axis class.

Healing-focused peptides like BPC-157 and TB-500 are in a different risk category. Preclinical data raises questions about tumour angiogenesis rather than IGF-1 mediated proliferation. The practical answer is the same: active cancer is a "wait" signal, not a "proceed" signal.

Pregnancy and breastfeeding are absolute contraindications#

Across the peptide universe discussed on Syntho, none has adequate safety data in pregnancy or breastfeeding. Not BPC-157. Not CJC-1295 or Ipamorelin. Not any GLP-1 agonist. Not NAD+. Not GHK-Cu for topical or injection use.

The reasons are straightforward. Peptides influence hormonal, metabolic, and growth signalling. Those pathways are actively remodelling in pregnancy. Introducing an exogenous signal into a system already in high-stakes change has no credible risk-benefit ceiling. Clinical-trial populations have excluded pregnant and breastfeeding individuals for every compound in the Syntho catalogue.

The practical rule: if you are pregnant, planning to become pregnant in the next 6–12 months, or breastfeeding, no peptide protocol is appropriate until that window closes. This is a pause, not a permanent bar.

Uncontrolled diabetes and severe insulin resistance need specialist input#

Several peptide classes affect glucose metabolism directly:

• GH secretagogues (CJC, Ipamorelin, Tesamorelin, MK-677) can reduce insulin sensitivity and raise fasting glucose during cycles. In uncontrolled type-2 diabetes, that worsens an already decompensated system.
• GLP-1 agonists (semaglutide, tirzepatide, retatrutide) are pharmacotherapy for diabetes, but self-managed off-label use in someone with complex diabetes on multiple agents introduces hypoglycaemia risk when combined with insulin or sulfonylureas.
• MK-677 elevates fasting glucose more than any other GH-axis molecule in published trials and is a common reason protocols are discontinued.

The practical rule: well-controlled diabetes (HbA1c under ~7%, stable regimen) doesn't preclude every peptide, but it does mean working with the person's diabetes team rather than around them. Uncontrolled diabetes (HbA1c ≥ 8% or unstable regimen) is a pause signal.

Severe kidney or liver impairment changes the calculus#

Peptides that are cleared renally — which includes most of the healing and GH-axis class — require caution when estimated glomerular filtration rate (eGFR) drops below 60 mL/min/1.73m². Below 30 mL/min/1.73m², most protocols are contraindicated without specialist oversight.

The liver side is similar. Many peptides are metabolised hepatically; elevated ALT or AST without a known cause warrants investigation before adding an exogenous signal. Syntho's questionnaire screens for both and the engine applies dose modifiers for eGFR 60–90 and flags ALT/AST elevations for review.

Age, bloodwork, and the baseline that matters#

Two structural contraindications are less obvious but just as important.

Age under 21. Growth plates close at different ages in different people. Introducing exogenous growth-signalling molecules in a still-developing endocrine system is not supported by any credible clinical literature. Syntho does not support peptide protocols for anyone under 21.

No recent bloodwork. Without at minimum CBC, comprehensive metabolic panel, lipid panel, HbA1c, IGF-1 (for GH-axis protocols), and TSH, there's no baseline to measure response against and no way to screen for silent contraindications. Syntho's protocol engine requires a structured health profile and flags missing biomarkers as the strongest signal against proceeding.

How to think about your own contraindications#

Three questions filter out the majority of risk before any product decision:

1. Do you have active or recent (≤ 5 years) cancer, or a family history of medullary thyroid carcinoma or MEN 2? If yes, stop here and discuss with an oncologist or endocrinologist.
2. Are you pregnant, trying to conceive in the next 6–12 months, or breastfeeding? If yes, no peptide protocol is appropriate right now.
3. Do you have recent bloodwork (CBC, CMP, HbA1c, lipids, IGF-1, TSH) within the last 12 months? If no, bloodwork is the correct first step — not a product.

If all three screen clean, the class-specific contraindications above still apply. The Syntho questionnaire runs this check structurally, maps your answers against peptide-specific safety rules, and produces a protocol that respects them. The protocol engine is conservative by design: when a class-level risk factor is present, affected peptides are removed from the candidate list rather than dose-adjusted. That's deliberate.

Peptide therapy works for the people it's a fit for. The whole point of screening is knowing whether that's you.