What it is
Tirzepatide is a single peptide that activates two incretin receptors: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1. Eli Lilly built it on a 39 amino acid backbone with a C20 fatty diacid chain that binds albumin, which stretches its half-life to about five days and allows one injection a week.
It is sold as two brands with the same molecule:
- Mounjaro, approved in the US in 2022 for type 2 diabetes.
- Zepbound, approved by the US FDA on 8 November 2023 for chronic weight management in adults with obesity, or overweight with at least one weight-related condition.
It is an approved prescription medicine. It sits in this library as the benchmark for the newer multi-receptor compounds.
How it works
The GLP-1 side does what semaglutide does: it raises glucose-dependent insulin release, lowers glucagon, slows stomach emptying and reduces appetite.
The GIP side is what makes tirzepatide different. GIP is the other main incretin hormone, and adding it was Lilly's bet that a second incretin signal would add to the GLP-1 effect on glucose and weight. How much of tirzepatide's result comes from GIP itself, including any effect on fat tissue, is still being worked out.
Retatrutide takes the same approach one step further by adding the glucagon receptor.
What the research found
SURMOUNT-1
SURMOUNT-1 was the pivotal obesity trial. It randomised 2,539 adults with a BMI of 30 or more, or 27 or more with a weight-related complication, and no diabetes, to 5 mg, 10 mg or 15 mg weekly or placebo for 72 weeks. The first 20 weeks were dose escalation. Mean baseline weight was 104.8 kg.
| Arm | Mean weight change at 72 weeks | Lost 5% or more |
|---|---|---|
| 5 mg | -15.0% | 85% |
| 10 mg | -19.5% | 89% |
| 15 mg | -20.9% | 91% |
| Placebo | -3.1% | 35% |
Half of the 10 mg group and 57% of the 15 mg group lost 20% or more of their body weight, against 3% on placebo. All prespecified cardiometabolic measures improved.
Dose escalation
The Zepbound label starts at 2.5 mg weekly for four weeks, which is a tolerability step, then moves to 5 mg and up in 2.5 mg increments no faster than every four weeks, to a maintenance dose of 5, 10 or 15 mg. The GLP-1 titration tool shows the label schedules for each branded GLP-1.
Side effects reported in studies
Gastrointestinal events were the most common in SURMOUNT-1, mostly mild to moderate and concentrated in the escalation period. Adverse events led to stopping treatment in 4.3%, 7.1% and 6.2% of the 5, 10 and 15 mg groups, against 2.6% on placebo.
The Zepbound label lists nausea, diarrhoea, vomiting, constipation, abdominal pain, dyspepsia, injection site reactions, fatigue, hypersensitivity reactions, belching, hair loss and reflux as the common adverse reactions. It carries the class boxed warning: tirzepatide caused thyroid C-cell tumours in rats, and it is contraindicated with a personal or family history of medullary thyroid carcinoma or MEN 2. It should not be combined with other GLP-1 agonists. More on managing these in the side effect management guide.
Tirzepatide vs semaglutide and retatrutide
| Compound | Receptors | Pivotal result | Stage |
|---|---|---|---|
| Semaglutide 2.4 mg | GLP-1 | -14.9% vs -2.4% at 68 weeks (STEP-1) | Approved |
| Tirzepatide 15 mg | GLP-1, GIP | -20.9% vs -3.1% at 72 weeks (SURMOUNT-1) | Approved |
| Retatrutide | GLP-1, GIP, glucagon | 28.3% at 80 weeks (TRIUMPH-1) | Phase 3, not approved |
Each figure comes from a different trial, so treat this as orientation, not a head-to-head comparison.
Status in the Philippines
Mounjaro is sold at retail at Watsons and Rose Pharmacy as a prescription medicine. No Philippine registration has been identified for Zepbound. Prescribing belongs with a licensed Philippine physician.
Watsons prices checked on its own pages on 26 August 2026:
| Mounjaro pen | PHP |
|---|---|
| 2.5 mg | 11,650 |
| 5 mg | 13,550 |
| 7.5 mg | 20,350 |
| 10 mg | 22,950 |
| 12.5 or 15 mg | 29,900 |
Each pen holds four weekly doses. See cost reality in the Philippines for what this means across a full course.
Tirzepatide has become the focus of local enforcement. On 14 March 2026 the Philippine College of Physicians advised against compounded and non-registered Ozempic and Mounjaro. On 13 May 2026 the NBI arrested two people in Tondo for selling unregistered tirzepatide online, citing RA 9711 (the FDA Act), RA 10175 (cybercrime) and RA 7394 (consumer protection). On 27 May 2026 the NBI and FDA Philippines raided a clinic in Pasig.
Local stock Tirzepatide 15mg at Primara Labs, delivered in Metro Manila and nationwide. Research use only.
Storage
The label calls for refrigeration at 2 to 8°C. It allows up to 21 days at room temperature up to 30°C, after which the pen must be discarded, and a pen kept at room temperature should not go back in the fridge. In the Philippines, 30°C is an ordinary afternoon, so a brownout or a hot car can take a pen out of range quickly. For reconstituted research vials, the tirzepatide reconstitution calculator covers the arithmetic.
