Do your genes affect incretin response?

Not everyone who takes semaglutide or tirzepatide loses the same amount of weight. The doses are identical, the lifestyle advice is consistent, yet the outcomes scatter widely. One underexplored explanation is that the receptors these drugs target are themselves genetically variable. A July 2026 review in the Journal of the Endocrine Society^[1] by Sandro La Vignera and Rosita A. Condorelli asks whether variants in the GLP-1 receptor gene (GLP1R) and the GIP receptor gene (GIPR) are systematically shifting those outcomes.

What a receptor polymorphism actually means

A polymorphism is a common DNA variant, present in at least 1% of a population, that changes the amino-acid sequence of a protein or alters how much of it is made. When the affected protein is a drug's binding site, the pharmacological consequence can be significant: the receptor may bind the drug with different affinity, signal downstream with different efficiency, or be expressed in different quantities on cell surfaces. The incretin receptors, GLP-1R and GIPR, are the binding sites for the molecules that semaglutide and tirzepatide activate. Both are G-protein-coupled receptors that work, in part, by driving cyclic AMP (cAMP) production inside cells. A variant that changes cAMP output changes the size of the metabolic signal the drug is sending.

The GLP1R variants with the best evidence

The La Vignera review focuses on two GLP1R single-nucleotide polymorphisms (SNPs) with published data behind them. The first is rs6923761 (Gly168Ser), which substitutes a serine for glycine at position 168 of the receptor. Carriers of this variant show altered cAMP signalling, and the clinical literature has connected it to differences in HbA1c reduction and body-weight response in patients on GLP-1 agonist therapy^[1]. A 2025 pre-specified analysis from the RISING STARS randomised trial (Mashayekhi et al., Journal of Endocrinology; PMID 41042549)^[2] tested this directly in adults with obesity and pre-diabetes on liraglutide. The result was a "dual impact": variant carriers lost significantly more weight with each copy of the allele, but showed diminished metabolic benefit on fasting insulin, C-peptide, glucagon, and HOMA-IR. Losing more weight is good; losing less glycaemic benefit is a complication that matters for the type-2 diabetes use case.

The second GLP1R variant, rs10305492 (Ala316Thr), also appears in the La Vignera analysis as a modifier of receptor-level cAMP signalling. The published dataset for this SNP is thinner than for rs6923761, and the review calls for larger prospective studies before clinical conclusions can be drawn.

The GIPR variant most relevant to tirzepatide

Where the analysis becomes specifically relevant to tirzepatide is in the GIPR data. Tirzepatide is a dual GIP and GLP-1 receptor agonist, meaning it acts on both receptors. The variant under review is GIPR rs1800437 (Glu354Gln), a substitution at position 354 of the GIP receptor protein. According to the La Vignera review^[1], this SNP occurs at roughly 20% frequency in European populations, and reaches around 22% in Italian cohorts. Carriers show an altered incretin effect and face a meaningfully elevated risk of nausea and vomiting on tirzepatide: the odds ratio for those adverse events is 1.83 compared to non-carriers. That is not a small signal. Gastrointestinal tolerability is already the main reason patients discontinue tirzepatide in trials, and a near-doubling of GI event odds in a fifth of the European population is clinically worth knowing.

What this does not yet tell us

The review is honest about its limits. The rs1800437 odds ratio comes from a constrained dataset; the authors call explicitly for prospective studies in Italian and broader Mediterranean cohorts before this translates into prescribing guidance. There is also the question of what to do with the information in practice. Pharmacogenomic testing for incretin-response variants is not part of any approved prescribing pathway in the EU, the UK, or the US as of this writing. No guideline from the European Society of Endocrinology, the American Diabetes Association, or NICE has recommended GLP1R or GIPR genotyping before starting semaglutide or tirzepatide. The science is in the "known unknown" category: the signal exists, but the infrastructure to act on it routinely does not.

A broader 2026 systematic review (Jayathilaka et al., Clinical Pharmacology in Drug Development; PMID 41238444)^[3] situates the GLP1R and ARRB1 variants alongside indirect pharmacogenomic effects from CYP2D6, which modifies metabolism of co-administered drugs like antidepressants and beta-blockers. The picture that emerges is that response variability in GLP-1 agonist therapy is polygenic and probably includes drug-drug-gene interactions, not just receptor-level variation. That complexity is part of why a simple "test before prescribing" policy is still some years away.

Practical context for readers of PeptideMethods

If you are following the tirzepatide evidence base, the La Vignera review is a useful reminder that trial averages (20.9% weight loss at 72 weeks in SURMOUNT-1; 6.5 percentage points ahead of semaglutide in SURMOUNT-5) describe populations, not individuals. The person who loses 8% on maximum-dose tirzepatide while their peer loses 30% may not be doing anything differently. Their receptors may simply be built differently. This is not an argument against the drug. It is context for managing expectations and for taking seriously the work being done to characterise responder populations before the next generation of incretin compounds reaches the clinic.

The editorial team will update this article as prospective pharmacogenomic data emerges. Nothing here constitutes prescribing guidance. Decisions about starting, continuing, or stopping any incretin therapy belong with a clinician who knows your history.