Why GLP-1 Drugs Work Better for Some People Than Others
If you are wondering why GLP-1 drugs work better for some people, the short answer is this: genetics probably plays a part, but it is not the main practical explanation in most cases. The new Nature paper suggests common variants in GLP1R and GIPR influence weight loss and nausea or vomiting, but the effects are modest, and non-genetic factors such as drug choice, dose, duration, sex, diabetes status and tolerability still matter greatly [1]. In this article I will cover three things: what the paper actually found, what most coverage gets wrong, and what patients should do over the first 2 weeks, weeks 3–6, and beyond 6 weeks if treatment seems disappointing. This applies to people using semaglutide or tirzepatide for weight management; it is not a substitute for individual prescribing advice.
The short answer
The paper is worth reading. It analysed 27,885 people who reported using semaglutide or tirzepatide based drugs and found a significant association between a GLP1R variant and greater weight-loss efficacy, roughly 0.76 kg more weight loss per effect allele. It also found associations between receptor-gene variation and nausea or vomiting, with a GIPR signal that appeared particularly relevant to tirzepatide [1].
The paper itself says the genetic effect sizes are modest [1]. Independent expert reaction has been similarly cautious: current evidence does not justify routine clinical decision-making based on these variants, because non-genetic factors still explain more of the observed variation than the common variants identified so far [2].
What the new Nature paper found
The cohort was large, but not broadly representative. Participants were mostly female (82.4%), median age was 52, and 78.3% were of European ancestry [1]. The main efficacy analysis was then conducted in a European-ancestry subset for GWAS purposes [1].
The study found that treatment efficacy depended strongly on sex, drug type, time on treatment and dose, with age having a weaker but still measurable effect [1]. Women tended to lose more than men before correction, tirzepatide performed better than semaglutide on average, and type 2 diabetes status predicted lower weight-loss efficacy [1]. Those findings are consistent with prior obesity-drug trial and real-world evidence [3–6].
The study also compared self-reported data with electronic health record data in a smaller subgroup. Self-reported weight loss looked materially larger than EHR-recorded weight loss, with a median change in BMI of -11.8% in surveys versus -5.79% in EHR data [1]. The discrepancy narrowed in the subgroup with both data sources, but did not disappear [1].
Why the paper matters
It matters because the biology is plausible. A receptor-gene signal for a drug acting on that receptor is much more interesting than a remote association with no obvious mechanistic relationship. It matters because the sample size is unusually large for this kind of obesity pharmacogenetic work [1]. And it matters because clinicians already know, from ordinary practice and from major trials, that response to GLP-1-based therapy varies widely [3–6].
In STEP 1, semaglutide 2.4 mg produced large mean weight loss, but with obvious inter-individual spread [3]. In SURMOUNT-1, tirzepatide produced larger mean weight loss again, but not uniformly [4]. In SURMOUNT-5, tirzepatide also outperformed semaglutide head-to-head on average [6]. None of that means every person will behave like the mean trial participant.
The paper therefore addresses a real problem. It just does not solve it. In obesity medicine, the field likes single elegant explanations more than it likes awkward clinical reality.
What most articles miss
Response is not just a receptor story: People want a neat answer. The paper offers one candidate explanation: receptor genetics. But the study’s own modelling shows response depends strongly on sex, drug type, dose and time on treatment [1]. In other words, even within the paper, the answer is already wider than DNA.
The phenotype is noisy: This is not a tightly run RCT with investigator-measured outcomes. Much of the study depends on self-report. The EHR comparison showed materially lower recorded BMI change than the survey data [1]. Sophisticated genetics does not cancel out soft outcome measurement.
Effect size is not the same as clinical usefulness: An extra 0.76 kg per allele is interesting scientifically [1]. It is not yet enough to change routine prescribing in a clinic tomorrow morning. That is why external experts have urged caution [2].
The question patients ask is different: Patients are rarely asking, “What is my pharmacogenomic profile?” They are asking, “Why is this not working for me?” That question may reflect inadequate titration, shortened treatment due to side effects, unrealistic time expectations, type 2 diabetes, poor tolerance, poor food structure, or a switch decision made too late.
Maintenance and discontinuation are barely addressed: The study is about on-treatment efficacy and side effects. It says very little about what happens when appetite returns, what determines durable maintenance, or how to manage people whose initial success is followed by a plateau, rebound, or loss of tolerability. That is a major clinical omission.
What the study can and cannot tell us
What it can tell us
It can tell us that common genetic variation in receptor pathways contributes to inter-individual variability in efficacy and side effects [1].
It can also tell us that non-genetic factors matter a great deal. The paper’s linear model explained about 21.4% of variance in percentage BMI loss using non-genetic variables such as age, sex, starting BMI, drug type, dose and time on treatment [1]. Type 2 diabetes status also materially reduced expected weight-loss response [1].
What it cannot tell us
It cannot tell us that genetic testing should now be routine before starting semaglutide or tirzepatide.
It cannot tell us that a disappointing response is “in your genes”.
It cannot tell us that severe nausea is a useful sign of success.
And it cannot tell us what the right clinician should do for the individual patient sitting in front of them next week.
Medicine is only improved when the association changes a real decision better than careful history and follow-up.
Strong evidence, weak evidence, and inference
Strong evidence
There is strong evidence that semaglutide and tirzepatide reduce body weight in appropriately selected adults with overweight or obesity, and that tirzepatide produces greater mean weight loss than semaglutide on average in head-to-head evidence [3–6]. There is also strong evidence that both drugs have important gastrointestinal adverse effects and labelled warnings, including contraindications related to medullary thyroid carcinoma / MEN2 and warnings around acute pancreatitis, gallbladder disease, volume depletion and severe GI reactions [7,8].
Moderate evidence
There is now moderate evidence that common receptor-gene variants contribute to variability in GLP-1 response and side effects [1]. Moderate, because the biology is plausible and the dataset is large, but the phenotype quality and generalisability are limited.
Inference / clinical experience
It is reasonable to infer that some people currently labelled “non-responders” are not true biological non-responders at all. They may be under-dosed, inadequately followed, poorly supported through side effects, switched too late, or treated without a proper plan for protein intake, food structure, symptom monitoring and escalation thresholds. That inference is clinically sensible, but it is still an inference.
What to do if your GLP-1 drug is not working well
The first step is not panic, and it is not buying a gene test. The first step is to ask a more disciplined set of questions:
Are you on the intended drug and the intended dose?
Have you been on that dose for long enough?
Have side effects limited escalation?
Was the treatment started for obesity, diabetes, or both?
Are you comparing your progress with trial averages that do not fit your starting point?
Has nausea produced chaotic eating rather than therapeutic appetite control?
Have you plateaued after initial benefit, or never meaningfully responded at all?
Is there a good reason to consider switching from semaglutide to tirzepatide, given average superiority of tirzepatide in obesity trials [4,6]?
A practical timeline
First 2 weeks
Focus on tolerance. It is usually too early to decide you are a non-responder. Early questions are about side effects, hydration, meal size, constipation, nausea, vomiting, and whether the patient is able to remain on treatment safely [7,8].
What to expect: appetite may fall before substantial scale change appears. GI symptoms may be present but should not simply be romanticised as proof of efficacy.
What to do: document dose, timing, appetite change, side effects, bowel habit, hydration, and early weight trend.
Weeks 3–6
This is the period in which poor tolerance can quietly become poor treatment. If dose escalation is repeatedly interrupted by nausea, vomiting or poor intake, the issue may be tolerability rather than lack of biological effect [7,8].
What most articles miss: some people stop progressing because they cannot actually stay on a useful dose long enough.
What to do: review whether the chosen drug is still appropriate, whether the escalation schedule is realistic, and whether the person is developing a workable eating structure rather than simply eating less unpredictably.
Beyond 6 weeks
This is where the management question becomes sharper. If a patient has had enough time at an effective dose, with tolerable side effects and reasonable adherence, but still shows weak response, then it becomes more reasonable to think about genuine low responsiveness, type 2 diabetes-related attenuation of weight-loss effect, or whether another agent may be better [1,4,6].
What to do: review drug choice, dose exposure, side-effect burden, diabetes status, weight trend, body composition trajectory if available, and whether continuing, switching or stopping is medically defensible.
When to involve your clinician
Speak to your clinician promptly if:
you cannot escalate because nausea or vomiting is persistent
you have minimal benefit despite an apparently adequate dose and duration
you have type 2 diabetes and your expectations were based on obesity-trial averages
you are considering switching drugs
you are thinking about stopping because of poor tolerance or poor response
you are getting weight loss at the cost of chaotic intake, weakness or functional decline
When to seek urgent help
Seek urgent medical assessment if you develop:
severe or persistent vomiting with inability to keep fluids down
signs of dehydration
severe abdominal pain, especially if persistent or associated with vomiting
symptoms concerning for gallbladder disease or pancreatitis
allergic symptoms such as facial swelling, breathing difficulty or widespread rash
These red flags matter more than genomics [7,8].
Bottom line
This is a useful paper, and it deserves attention. It shows that common variants in GLP1R and GIPR are associated with differences in GLP-1 efficacy and side effects [1]. But it does not support the louder conclusion many people will want to draw: that obesity prescribing is now becoming a straightforward genomic sorting exercise. That is the point at which precision medicine becomes precision medicine theatre. The better conclusion is narrower and more clinically honest. Genetics probably explains some of why GLP-1 drugs work better for some people. But most of the practical work still lies elsewhere: in dose, duration, tolerability, drug selection, diabetes status, and proper clinical interpretation.
FAQs
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Because response depends on more than one thing. Genetics may play a part, but dose, drug choice, time on treatment, side effects, sex, and diabetes status also matter [1–6].
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Not at present. The paper is important, but current evidence does not support routine prescribing decisions based on these variants alone [1,2].
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Possible reasons include not yet reaching or tolerating an effective dose, having type 2 diabetes, stopping escalation because of side effects, or simply needing more time before judging response [1,3,7].
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On average, tirzepatide produces greater weight loss in major trials and head-to-head evidence, though that does not guarantee it will be superior for every individual [4,6].
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Not necessarily. Side effects may coexist with benefit, but severe nausea or vomiting can also make treatment unsustainable and clinically unhelpful [1,7,8].
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Do not declare failure too early. In the first 2 weeks, the main priorities are tolerability, hydration, bowel habit, appetite change and whether you can remain on treatment safely [7,8].
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Usually not in the first few weeks. It becomes a more reasonable discussion after adequate dose exposure, enough time on treatment, and a clear review of side effects, expectations and actual response [4,6].
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Probably yes, but that category is often used too loosely. Some apparent non-response is really under-dosing, poor tolerance, unrealistic timing, or weak clinical follow-up rather than a pure biological failure.
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If side effects are limiting dose escalation, if you have had little benefit after an adequate treatment period, or if you are thinking about switching or stopping, involve your clinician early.
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Seek urgent assessment for severe persistent vomiting, inability to keep fluids down, dehydration, severe abdominal pain, or symptoms suggesting pancreatitis, gallbladder disease or serious allergy [7,8].
References
Su QJ, Ashenhurst JR, Xu W, et al. Genetic predictors of GLP1 receptor agonist weight loss and side effects. Nature. 2026. URL: https://www.nature.com/articles/s41586-026-10330-z
Science Media Centre. Expert reaction to paper about genetic predictors of GLP1 receptor agonist weight loss and side effects. 2026. URL: https://www.sciencemediacentre.org/expert-reaction-to-paper-about-genetic-predictors-of-glp1-receptor-agonist-weight-loss-and-side-effects/
Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med. 2021;384(11):989-1002. URL: https://www.nejm.org/doi/full/10.1056/NEJMoa2032183
Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387(3):205-216. URL: https://www.nejm.org/doi/full/10.1056/NEJMoa2206038
Rodriguez PJ, et al. Semaglutide vs tirzepatide for weight loss in adults with overweight or obesity. JAMA Intern Med. 2024. URL: https://pmc.ncbi.nlm.nih.gov/articles/PMC11231910/
Aronne LJ, et al. Tirzepatide as compared with semaglutide for the treatment of obesity. N Engl J Med. 2025. URL: https://www.nejm.org/doi/full/10.1056/NEJMoa2416394
U.S. Food and Drug Administration. Wegovy (semaglutide) prescribing information. Revised 02/2026. URL: https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/215256s033lbl.pdf
U.S. Food and Drug Administration. Zepbound (tirzepatide) prescribing information. Revised 01/2026. URL: https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/217806s002lbl.pdf
National Institute for Health and Care Excellence. Semaglutide for managing overweight and obesity. TA875. 2023. URL: https://www.nice.org.uk/guidance/ta875
National Institute for Health and Care Excellence. Tirzepatide for managing overweight and obesity. TA1026. 2024. URL: https://www.nice.org.uk/guidance/ta1026
NHS England. Weight management injections - obesity. 2026. URL: https://www.england.nhs.uk/ourwork/prevention/obesity/medicines-for-obesity/weight-management-injections/
Loos RJF. Genetics reveal why people respond differently to GLP-1 weight-loss drugs. Nature. 2026. URL: https://www.nature.com/articles/d41586-026-00905-1
