"How does Mounjaro actually work?" is the question I hear most often from patients considering weight loss treatment. It's a fair question, and it deserves a proper clinical answer, not a marketing summary. In this article I'll explain exactly what tirzepatide does in your body, what the trial data shows, and what you can realistically expect if you're prescribed it.

What is Mounjaro?

Mounjaro is the brand name for tirzepatide, a prescription-only injectable medication manufactured by Eli Lilly. It was first licensed in the UK for type 2 diabetes in 2022, and then received MHRA approval for weight management in November 2023. It is the first dual-action GLP-1/GIP receptor agonist to be licensed for obesity in the UK.

Mounjaro is administered as a once-weekly subcutaneous injection using a prefilled KwikPen. You inject it into your stomach, thigh, or upper arm, rotating the site each week.

The two-hormone mechanism: why Mounjaro is different

GLP-1 and GIP are gut hormones that your body releases naturally after eating. Older weight loss injections like Wegovy and Saxenda mimic GLP-1 only. Mounjaro is the first medication to mimic both GLP-1 and GIP simultaneously. Scientists originally doubted whether dual action would improve outcomes compared to GLP-1 alone, but the clinical data proved otherwise convincingly.

GLP-1 activation

  • Signals the hypothalamus (the brain's appetite control centre) that you're full
  • Slows gastric emptying, food moves more slowly from your stomach
  • Stimulates the pancreas to release insulin when blood glucose rises
  • Suppresses glucagon, the hormone that raises blood sugar between meals
  • Reduces cravings and "food noise" via brain reward pathways

GIP activation

  • Promotes fat metabolism and reduces fat storage in fat cells
  • Enhances the appetite-suppressing effects of GLP-1
  • Improves insulin sensitivity in peripheral tissues
  • Acts on the central nervous system to regulate energy balance

How tirzepatide works in your body

GLP-1 Receptor

Appetite Control

Signals the hypothalamus to reduce hunger, slows gastric emptying, and quietens food cravings.

↓ Appetite

GIP Receptor

Fat Metabolism

Promotes fat breakdown in fat cells, improves insulin sensitivity, and enhances GLP-1's appetite-suppressing effects.

↓ Fat storage

Dual Action

Superior Results

Tirzepatide is the only medication to activate both receptors simultaneously, producing greater weight loss than GLP-1 medications alone.

−22.5% body weight

Source: Jastreboff AM et al. N Engl J Med 2022 (SURMOUNT-1)

"In the landmark SURMOUNT-1 randomised controlled trial, patients on tirzepatide 15mg lost an average of 22.5% of their starting body weight over 72 weeks. Diet and exercise alone produced 2.4% weight loss over the same period."

Source: Jastreboff AM et al. N Engl J Med 2022;387:205 to 216.

How quickly does Mounjaro work?

Most patients notice reduced appetite within the first week of treatment. Meaningful, measurable weight loss typically begins at 4 to 8 weeks. In the SURMOUNT-1 trial, results at 72 weeks were dose-dependent: 5mg produced an average of 16% body weight loss, 10mg produced 21.4%, and 15mg produced 22.5%.

At the highest dose, 36.2% of patients lost 25% or more of their body weight, and 91% of those on 10mg or 15mg lost at least 5%. These results are comparable to bariatric surgery in some published analyses.

Dosing and titration

Treatment starts at 2.5mg once weekly. The dose increases by 2.5mg every four weeks according to tolerability, up to a maximum of 15mg. There is no requirement to reach the maximum dose. Many patients achieve excellent results at 10mg or even 7.5mg.

The low starting dose is deliberate: it minimises gastrointestinal side effects while your body adjusts. Rushing the titration schedule is the leading cause of difficult side effects, which is why your pharmacist will review your progress before each dose increase.

Who is Mounjaro suitable for?

Mounjaro is suitable for adults with a BMI of 30 or above, or a BMI of 27 or above with at least one weight-related condition such as hypertension, type 2 diabetes, dyslipidaemia, or obstructive sleep apnoea.

Contraindications

Mounjaro is not suitable for patients with a personal or family history of medullary thyroid carcinoma (MTC), Multiple Endocrine Neoplasia syndrome type 2 (MEN 2), a history of pancreatitis, known allergy to tirzepatide, type 1 diabetes, or pregnancy.

An important note for patients taking oral contraceptives: you should use an additional barrier method of contraception for four weeks after starting Mounjaro and for four weeks after each dose increase. Tirzepatide slows gastric emptying and may reduce the absorption of oral medications, including the contraceptive pill.

Side effects

The most common side effects are gastrointestinal and occur mainly during dose increases. Nausea is very common, affecting more than 1 in 10 people, approximately 17% to 27% depending on dose. Diarrhoea, vomiting, and constipation are also common. Most of these symptoms improve as your body adjusts, typically over 4 to 8 weeks at each dose level.

Rare but serious side effects include pancreatitis and gallbladder disease. Any severe or persistent abdominal pain warrants prompt medical attention.

When to see your pharmacist or doctor

  • Severe or persistent nausea, vomiting, or abdominal pain
  • Symptoms of pancreatitis: severe upper abdominal pain radiating to the back
  • Planning pregnancy or discovered you are pregnant
  • Taking oral contraceptives (additional precautions required)
  • Not losing weight after 3 months at an appropriate dose

References

  1. Jastreboff AM et al. N Engl J Med 2022;387:205 to 216 (SURMOUNT-1).
  2. MHRA Public Assessment Report: Mounjaro (tirzepatide), November 2023.
  3. NICE Technology Appraisal TA1026 (2024).
  4. Mounjaro Summary of Product Characteristics, Eli Lilly, 2024.

Reviewed by Jimah Alsaai MPharm, GPhC 2232652, Superintendent Pharmacist, Solira.