Who shouldn't take peptides: contraindications guide

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 and has been shown to promote 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). Research suggests the relationship between IGF-1 and cancer is more complex than a simple linear risk, but it is not neutral: in roughly 430,000 women, Murphy et al. (2020) found higher circulating IGF-1 associated with higher breast cancer risk, in both observational and Mendelian randomization analyses. Nothing in that literature gives clearance for people with active cancer.

The rule Klarovel's engine applies follows the tesamorelin label, which contraindicates active malignancy and asks for a benefit-risk decision after a treated, stable one. Active cancer blocks every GH secretagogue and every pro-angiogenic peptide. A past cancer with treatment completed blocks the strongly pro-angiogenic peptides (TB-500, Dihexa) and Melanotan II, and puts the GH-axis class under caution, which means oncology clearance first. A family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 is a different issue: it is a labeled contraindication for GLP-1 drugs, not for GH secretagogues (see the callout below).

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, and Klarovel's engine blocks both with active cancer.

Pregnancy and breastfeeding are absolute contraindications#

Across the peptide universe discussed on Klarovel, 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 Klarovel catalogue.

The practical rule: if you are pregnant, planning a pregnancy, or breastfeeding, no peptide protocol is appropriate until that window closes. Klarovel's engine blocks every peptide for all three answers. 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 raises fasting glucose and lowers insulin sensitivity in published trials, which is why Klarovel's engine blocks it outright at an HbA1c of 5.7% or higher, a fasting glucose of 100 mg/dL (5.6 mmol/L) or higher, or a fasting insulin of 12 µIU/mL or higher, and with type 1 diabetes.

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, GH-axis peptides and GHK-Cu 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. Klarovel's questionnaire screens for both. When kidney or liver disease is reported, the engine halves the curated dose band for every peptide except the GLP-1 drugs, whose labels call for no renal or hepatic adjustment. When an eGFR value is available, it cuts GH-axis doses by 25% below 60 and blocks GH-axis peptides and GHK-Cu below 30. An ALT above about three times the upper reference limit blocks oral 5-Amino-1MQ, and a moderate ALT elevation puts it under caution.

Age, bloodwork, and the baseline that matters#

Two structural contraindications are less obvious but just as important.

Young age. Klarovel builds protocols only for adults 18 and over. Growth plates close at different ages in different people, and introducing exogenous growth-signalling molecules into a still-developing endocrine system is not supported by credible clinical literature, so growth-axis peptides deserve extra caution in younger adults too.

Bloodwork as a sharpener, not a gate. A recent CBC, comprehensive metabolic panel, lipid panel, HbA1c, IGF-1 (for GH-axis protocols) and TSH give a baseline to measure response against and can surface contraindications the questionnaire cannot, such as a high IGF-1 or a prediabetic HbA1c. Klarovel's engine builds the protocol from the structured health profile; when lab values are missing, it lists the ones that would sharpen the next cycle rather than refusing to proceed.

Contraindications are about who should not start. For what happens to people who do, organised by compound and severity, see the peptide side effects guide.

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 know your metabolic and kidney baseline? A recent panel (CBC, CMP, HbA1c, lipids, IGF-1, TSH) is optional, but it is the only way to catch the silent contraindications above.

If all three screen clean, the class-specific contraindications above still apply. The Klarovel 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: it grades each match as block, caution, monitor or dose reduction. A blocked peptide is removed from the candidate list and shown to you with the reason; the lower grades keep the peptide with a warning, a monitoring schedule or a reduced dose. That's deliberate, and it is part of the framework documented on Klarovel's disclosures page.

Peptide therapy works for the people it's a fit for. The whole point of screening is knowing whether that's you, which is the starting point for the broader question of whether peptides are safe.