Mounjaro (tirzepatide) and Orlistat represent two fundamentally different approaches to weight loss. One is a cutting-edge injectable hormone therapy; the other is a well-established oral medication that blocks fat absorption. Here is how they compare.
Mechanism of Action
Mounjaro targets GIP and GLP-1 receptors in the brain and gut, reducing appetite and slowing gastric emptying. Weight loss comes primarily from eating significantly less.
Orlistat (brand names Xenical or Alli) inhibits pancreatic lipase, the enzyme that breaks down dietary fat. Approximately 30% of the fat you eat passes through undigested, reducing calorie absorption.
Efficacy
- Mounjaro: 15–22% average body weight loss in clinical trials.
- Orlistat: 3–5% average body weight loss beyond diet and exercise alone.
The difference is substantial. Mounjaro delivers roughly four to six times the weight loss of Orlistat in controlled studies.
Convenience and Administration
- Mounjaro: Once-weekly subcutaneous injection via pre-filled pen.
- Orlistat: One capsule taken orally with each main meal (up to three times daily).
Some patients prefer oral medication over injections. However, Orlistat must be taken with every meal containing fat, which requires more day-to-day adherence.
Side Effects
Mounjaro commonly causes nausea, vomiting and constipation, especially during titration. These usually improve with time.
Orlistat has distinctive gastrointestinal effects: oily or fatty stools, flatulence, faecal urgency and oily spotting. These are directly related to fat intake—the more fat you eat, the worse the side effects. Many patients find these effects socially embarrassing and difficult to tolerate long term.
Cost Comparison
- Orlistat (Alli 60 mg): Available over the counter from UK pharmacies for around £30–£50 per month.
- Orlistat (Xenical 120 mg): Prescription only, approximately £40–£80 per month privately.
- Mounjaro: £150–£300 per month depending on dose and pharmacy.
Orlistat is considerably cheaper, which makes it accessible for people who cannot afford or are not eligible for newer treatments.
Who Is Each Best For?
Orlistat may suit people with a modest amount of weight to lose (BMI 28–34), those who prefer oral tablets, or those on a tight budget. Mounjaro is better suited to people with a BMI of 30+ (or 27+ with comorbidities) who need significant weight loss and are comfortable with weekly injections.
Compare prices for both medications across UK pharmacies using MedsLens to ensure you get the best value for your chosen treatment.