Mounjaro & Wegovy: Complete Guide to NHS Weight Loss Injections
Discover how Mounjaro and Wegovy weight loss injections work, their effectiveness, eligibility on the NHS, costs, side effects, and BMI guidelines for healthy weight management.
Weight loss injections like Mounjaro and Wegovy have gained huge popularity in the UK, especially as part of NHS obesity treatment programs. These medications, originally developed for diabetes, are now being used to help people lose weight by suppressing appetite and improving metabolic balance. This guide explains how they work, who can get them on the NHS, their costs, side effects, and key health considerations.
What Are Mounjaro and Wegovy?
Two major weight loss drugs currently available are:
- Wegovy (Semaglutide) – marketed under the brand name Wegovy.
- Mounjaro (Tirzepatide) – sold as Mounjaro.
Both are administered as weekly self-injections using pre-filled pens that can be applied to the upper arm, thigh, or stomach. They mimic natural hormones that make people feel fuller, reducing appetite and calorie intake.
Mounjaro also improves metabolism and helps regulate energy balance, making it potentially more effective for some patients. Treatment usually starts with a low dose, gradually increasing to a maintenance dose.
How Effective Are They for Weight Loss?
People usually notice weight loss within a few weeks of starting treatment.
A head-to-head clinical trial found that:
- Mounjaro achieved an average 20% weight loss after 72 weeks.
- Wegovy achieved around 14% weight loss over the same period.
The trial, funded by Eli Lilly (Mounjaro's manufacturer), involved 750 obese participants with an average starting weight of 113kg.
Other studies suggest:
- Mounjaro users may lose up to one-fifth of their body weight.
- Wegovy users could lose over 10% of their weight in one year when combined with diet, exercise, and behavioural support.
However, weight regain is common after stopping treatment, so ongoing lifestyle changes are essential.
NHS Eligibility for Weight Loss Drugs
Wegovy
Available on the NHS in England, Wales, and Scotland.
Eligibility:
- BMI ≥ 35 and at least one weight-related health condition (e.g., high blood pressure).
- Some overweight patients with cardiovascular disease may also qualify.
- Prescription is limited to a maximum of two years alongside diet and exercise plans.
Mounjaro
Available through specialist NHS clinics since March 2025 and from some GPs since June 2025.
Eligibility:
- BMI ≥ 40 (or ≥ 37.5 for minority ethnic backgrounds).
- Must have four out of five conditions: type 2 diabetes, high blood pressure, cardiovascular disease, high cholesterol, obstructive sleep apnoea.
- Comes with regular health check-ups, exercise support, and healthy eating advice.
The National Institute for Health and Care Excellence (NICE) estimates that up to 3.4 million people could eventually qualify, but full rollout may take over a decade.
Private Purchase Costs
Wegovy and Mounjaro can be purchased privately from supermarkets, pharmacies, and clinics.
- Typical cost: £200–£300 per pen, depending on the dose.
- Must only be bought from regulated sources.
New UK rules require online pharmacies to verify BMI and medical history via in-person or video consultations to prevent misuse by people with healthy weight or eating disorders.
Side Effects
Common side effects include:
- Nausea
- Vomiting
- Bloating
- Constipation or diarrhoea
- Occasional hair loss
Most symptoms improve over time, but misuse can cause serious complications such as gallbladder problems, kidney issues, depression, or rare pancreatic problems.
Pregnancy Warnings
The MHRA advises women not to take these drugs:
- During pregnancy
- While trying to conceive
- While breastfeeding
They may also reduce the effectiveness of contraceptive pills, so alternative birth control may be needed.
Understanding Obesity & BMI
Obesity is defined as having excess body fat, and affects around 1 in 4 UK adults.
BMI (Body Mass Index) is calculated as:
weight (kg) ÷ height² (m²)
BMI Categories for most adults:
- Underweight: < 18.5
- Healthy: 18.5 – 24.9
- Overweight: 25 – 29.9
- Obese: 30 – 39.9
- Severely Obese: 40+
For people from Asian, Chinese, Middle Eastern, Black African, or African-Caribbean backgrounds:
- Overweight: 23 – 27.4
- Obese: 27.5+
While BMI has limitations, it’s still a useful tool for most adults to assess healthy weight ranges.
Conclusion
Mounjaro and Wegovy offer powerful medical support for weight loss, but they are not magic solutions. Long-term success depends on healthy eating, regular exercise, and lifestyle changes. If you think you may qualify, speak with your GP or a certified weight loss clinic to explore your options safely.