What GIP adds to GLP-1
GLP-1 (glucagon-like peptide-1) is well-characterized: slows gastric emptying, increases satiety, suppresses glucagon, augments insulin release. Mostly an appetite-and-glucose hormone.
GIP (glucose-dependent insulinotropic polypeptide) adds two distinct effects on the fat-loss side: improved insulin sensitivity in adipose tissue (helps the body burn stored fat) and enhanced fat oxidation during energy deficit. The dual agonist effect is not additive in a simple way — it appears to be synergistic, which is the most likely explanation for SURMOUNT-5 outcomes.
Head-to-head: SURMOUNT-5
Published 2024 in NEJM. 751 adults with BMI ≥30, randomized to tirzepatide 15 mg or semaglutide 2.4 mg, 72 weeks of treatment. This is the only large head-to-head trial of the two drug classes to date.
| Outcome | Tirzepatide 15mg | Semaglutide 2.4mg |
|---|---|---|
| Mean weight loss (%) | −20.2% | −13.7% |
| Achieved ≥15% loss | 64% | 40% |
| Achieved ≥25% loss | 32% | 16% |
| Waist circumference | −18.4 cm | −13.0 cm |
| Discontinuation (adverse events) | 10.4% | 7.6% |
Translation: tirzepatide produced 47% more relative weight loss with a 3-percentage-point higher dropout rate. Most clinicians read this as "tirzepatide first-line if tolerated, semaglutide if cost or side effects favor it."
When to choose each
| Situation | Better choice | Why |
|---|---|---|
| Goal: maximum weight loss | Tirzepatide | 47% greater relative loss in head-to-head |
| Goal: cardiovascular risk reduction | Semaglutide | SELECT trial — only GLP-1 with explicit CV outcome data |
| Sleep apnea + obesity | Tirzepatide (Zepbound) | Only GLP-1 FDA-approved for OSA |
| Higher GI sensitivity | Semaglutide | Slightly lower side-effect rate |
| Insurance covers one but not other | Covered one | Don't pay 4x list price for marginal extra benefit |
| Prefer oral pill | Oral Wegovy or Foundayo | Both are GLP-1 only — no GIP oral yet |
What comes next: triple agonists
Retatrutide is the next pharmacology jump: a single molecule that activates GLP-1, GIP, and glucagon receptors. Phase 2 data (TRIUMPH series, 2024-2025) showed mean weight loss of ~24% at 48 weeks — close to bariatric-surgery range without surgery.
Phase 3 trials are ongoing; expected Lilly FDA filing 2026-2027. If approved on similar timeline to Foundayo, retatrutide could reach US patients in late 2027 or 2028. Until then, Zepbound remains the top-of-line option.
Compare providers carrying both classes
Some telehealth providers carry only semaglutide. Others carry tirzepatide + oral options + Foundayo. See which providers have the full formulary.
See top providers →Frequently asked questions
- What is the difference between GLP-1 and GLP-1/GIP medications?
- GLP-1 medications (semaglutide, orforglipron) activate only the GLP-1 receptor. GLP-1/GIP dual agonists (tirzepatide) activate both GLP-1 and GIP receptors. Dual agonism adds a second pathway for appetite regulation and energy expenditure.
- Why is tirzepatide more effective than semaglutide?
- In the SURMOUNT-5 head-to-head trial, tirzepatide produced 47% greater relative weight loss than semaglutide at 72 weeks. The mechanism: GIP receptor activation appears to enhance fat oxidation and improve adipose tissue function, on top of GLP-1's appetite effects.
- Should I choose tirzepatide over semaglutide?
- Not always. Tirzepatide is more effective on average but also has slightly higher GI side-effect rates and is typically more expensive. For patients meeting target weight loss on semaglutide, switching has minimal upside.
- Is Foundayo (orforglipron) GLP-1 or GLP-1/GIP?
- GLP-1 only. Foundayo is a small-molecule GLP-1 agonist — same single-receptor pathway as semaglutide, in oral pill form. Expect efficacy roughly between Rybelsus and injectable Wegovy.
- Are there other GLP-1 multi-agonists in development?
- Yes — retatrutide (GLP-1/GIP/glucagon triple agonist by Lilly) showed ~24% weight loss in phase 2 trials. Expected FDA filing 2026-2027. Other dual + triple agonists are in earlier stages.
Disclosure: Glpverdict is affiliate-funded but editorially independent. Trial data is summarized from primary publications (SURMOUNT-5 in NEJM, ATTAIN-1 NEJM, SURMOUNT-OSA NEJM). Clinical content reviewed by Jane Novak, MD, MPH.