Ask whether GLP-1 drugs cause cancer or prevent it and you can find a confident answer either way. Both come from real studies. The difference is in the kind of study and — the part most summaries leave out — what the drug was compared with.
This page reads the whole cancer record as of 25 September 2026: the labels, the two largest trial meta-analyses, and the largest US records study. Thyroid cancer, the one cancer on the labels, has its own page — the GLP-1 thyroid cancer warning — and is summarised here only where it sits in the wider picture.
What the labels say
Every current US label for a long-acting GLP-1 drug we have read — Wegovy, Ozempic, Zepbound, Mounjaro, Trulicity, Saxenda, Victoza, Foundayo — carries the same boxed warning: the drug caused thyroid C-cell tumours in rodents, and human relevance has not been determined. Each is contraindicated in people with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2.
No label claims a reduction in any cancer, and no label lists another cancer as a contraindication. That is the regulatory floor: one warning, from animal studies, and nothing else either way.
What the randomised trials show
Trials are the stronger evidence, because randomisation balances everything except the drug. Their weakness is time: the pooled trials had short follow-up and were not designed to count cancers, as the meta-analysis authors state.
Ko et al., Annals of Internal Medicine, 2026 — 48 placebo-controlled trials, 94,245 participants, 13 obesity-related cancers:
| Cancer | Odds ratio (95% CI) | Per 10,000 treated | Certainty |
|---|---|---|---|
| Thyroid | 1.37 (0.82–2.31) | 1 fewer to 9 more | moderate |
| Pancreatic | 0.84 (0.53–1.35) | 9 fewer to 6 more | moderate |
| Breast | 0.95 (0.60–1.49) | 10 fewer to 12 more | moderate |
| Kidney | 1.12 (0.78–1.60) | 5 fewer to 13 more | moderate |
| Colorectal, oesophageal, liver, gallbladder, ovarian, endometrial, myeloma, meningioma | — | — | low: may have little or no effect |
| Stomach | — | — | very uncertain |
The authors' conclusion: GLP-1 drugs "may have little or no effect on risk for obesity-related cancers", with longer studies needed. Results held for semaglutide and tirzepatide separately.
Silverii et al., Diabetes, Obesity and Metabolism, 2025 — 50 trials of at least 52 weeks, any comparator: no difference in overall cancer (odds ratio 1.05, 0.98–1.13); more thyroid cancer (1.55, 1.05–2.27), clearer in longer trials; more colorectal cancer (1.27, 1.03–1.57) — but only in the shorter trials. The authors' explanation is worth knowing: people with bowel side effects get investigated more, and investigation finds cancers that were already there. On that reading, a drug that causes bowel symptoms can raise the diagnosed rate of bowel cancer in short trials without causing any tumour.
What the big records study shows — and the comparator problem
Wang et al., JAMA Network Open, 2024 followed 1,651,452 people with type 2 diabetes in US electronic health records from 2005 to 2018 for up to 15 years, comparing those prescribed GLP-1 drugs with those prescribed insulin or metformin, after propensity matching.
Against insulin, GLP-1 drugs were associated with lower risk of 10 of 13 obesity-associated cancers — among them pancreatic (hazard ratio 0.41), liver (0.47), colorectal (0.54) and kidney (0.76). That is the finding most headlines report.
Against metformin, the same study found GLP-1 drugs associated with lower risk of none of the 13 — and with higher risk of kidney cancer (hazard ratio 1.54).
| Cancer | GLP-1 vs insulin | GLP-1 vs metformin |
|---|---|---|
| Kidney | 0.76 — lower | 1.54 — higher |
| Colorectal, gallbladder | lower | below 1, not significant |
| The other 10 of the 13 | 7 lower | none lower |
The same drug class, in the same database, points in opposite directions for kidney cancer depending on what it is compared with. Insulin is usually prescribed later in diabetes, to sicker people; metformin is usually first-line, to healthier ones. An observational comparison measures the drug and the kind of person who gets it, and matching cannot fully separate them. The study's own conclusion calls its findings "preliminary evidence". It also predates the obesity approvals, so it says nothing about people without diabetes.
Where the three kinds of evidence agree
- No evidence that GLP-1 drugs raise overall cancer risk in the trials.
- Thyroid is the one signal that recurs: a boxed warning from rodents, a raised odds ratio in one trial meta-analysis, a wide interval straddling no effect in the other, and no reduction against either comparator in the records study.
- No evidence strong enough to call these drugs cancer prevention. The protective associations depend on the comparator and come from observational data.
Other safety questions with their own evidence files are on this site: pancreatitis, long-term side effects, and the class-wide safety summary.
What this page cannot tell you
- Long-term effects. The pooled trials had short follow-up; none was designed around cancer.
- Anything about one person's risk. These are group estimates.
- Whether someone with a cancer history should use these drugs. That belongs with the oncologist and prescriber.
Sources
- Ko A et al. Risk for cancer with glucagon-like peptide-1 receptor agonists and dual agonists: a systematic review and meta-analysis. Ann Intern Med 2026;179(2):216–229. doi:10.7326/ANNALS-25-02237, abstract (PubMed 41359966).
- Silverii GA et al. GLP-1 receptor agonists and the risk for cancer: a meta-analysis of randomized controlled trials. Diabetes Obes Metab 2025;27(8):4454–4468. doi:10.1111/dom.16489, abstract (PubMed 40437949).
- Silverii GA et al. GLP-1 receptor agonists and risk of thyroid cancer: a systematic review and meta-analysis of randomized controlled trials. Diabetes Obes Metab 2024;26(3):891–900. doi:10.1111/dom.15382, abstract (PubMed 38018310).
- Wang L et al. Glucagon-like peptide 1 receptor agonists and 13 obesity-associated cancers in patients with type 2 diabetes. JAMA Netw Open 2024;7(7):e2421305. doi:10.1001/jamanetworkopen.2024.21305, abstract (PubMed 38967919).
- ZEPBOUND (tirzepatide) prescribing information, Eli Lilly, effective 2026-08-28 — DailyMed, boxed warning and section 4; the same warning read on the current Wegovy, Ozempic, Mounjaro, Trulicity, Saxenda, Victoza and Foundayo labels on 25 September 2026.