GLP1s

How GLP-1 Weight Loss Injections Work

TribeTeam

March 28, 2026

Searches for GLP-1 weight loss injections, Mounjaro and Wegovy, and how Semaglutide works have surged in the past two years. But beyond headlines and social media trends, what does the science actually say?

This evidence-based guide explains how these medications work, and what you should know before starting treatment.

What Are GLP-1 Weight Loss Injections?

GLP-1 (glucagon-like peptide-1) receptor agonists mimic a natural gut hormone involved in:

01	Appetite regulation
02	Blood glucose control
03	Slowing gastric emptying
04	Signalling satiety to the brain

They are licensed for weight management in people with obesity or weight-related medical conditions.

Two of the most prescribed options are:

Wegovy (Semaglutide) Mounjaro (Tirzepatide)

How Does Wegovy Work?

Wegovy contains Semaglutide, a medication that mimics the natural hormone GLP-1 (glucagon-like peptide-1). GLP-1 is released after eating and helps regulate appetite and blood sugar.

Wegovy works by:

Reducing hunger signals and helping you feel less driven to eat
Increasing satiety, so smaller meals feel more satisfying
Slowing stomach emptying, which can prolong fullness after meals
Helping regulate blood sugar levels
Reducing cravings and food noise for many users

This can help people eat less without relying solely on willpower. Wegovy is used as a once-weekly injection and works best when combined with nutrition, movement, and long-term behaviour change.

~15%

average body weight loss over 68 weeks in the STEP clinical trial programme

In the STEP clinical trial programme published in the New England Journal of Medicine, participants lost on average ~15% of body weight over 68 weeks when combined with lifestyle support.
(See: Wilding et al., 2021)

Backlink reference: Wilding JPH et al. "Once-Weekly Semaglutide in Adults with Overweight or Obesity." NEJM. 2021.

How Does Mounjaro Work?

Mounjaro contains Tirzepatide, which activates two hormone pathways in the body: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). These are natural gut hormones involved in appetite control, blood sugar regulation, and metabolism. By mimicking these hormones, Mounjaro helps to:

- Reduce appetite and lower hunger signals in the brain
- Increase feelings of fullness after eating
- Slow stomach emptying, helping you stay satisfied for longer
- Improve insulin sensitivity and blood sugar control
- Reduce food cravings in many people
Because it works on two pathways rather than one, Mounjaro may lead to a stronger appetite and metabolic effects for some individuals. It is used as a once-weekly injection and ideally alongside healthy lifestyle changes for long term weight loss.

~20%

average weight loss at higher doses in the SURMOUNT-1 trial

In the SURMOUNT-1 trial (also published in the New England Journal of Medicine), participants achieved ~20% average weight loss at higher doses.
(See: Jastreboff et al., 2022)

Backlink reference: Jastreboff AM et al. "Tirzepatide Once Weekly for the Treatment of Obesity." NEJM. 2022.

Who Is Eligible for GLP-1 Weight Loss Injections?

According to NICE and FDA guidance, these medications are generally prescribed for:

BMI ≥30 BMI ≥27 with weight-related conditions (e.g., hypertension, type 2 diabetes, dyslipidaemia)

Ref: Obesity management guidance from National Institute for Health and Care Excellence (NICE).

They are intended for chronic weight management and can be prescribed by private providers, such as TribElle or the NHS.

Eligibility for the NHS varies from area to area. Exact access depends on where you live in the UK and local NHS service capacity, but in general, the current criteria are:

Main NHS Eligibility Criteria for Weight Loss GLP-1s

Wegovy (England NICE guidance)

Typically for adults who have:

BMI of 35 kg/m² or above
and
At least one weight-related health condition, such as:
- Type 2 diabetes
- High blood pressure
- Sleep apnoea
- High cholesterol
- Cardiovascular disease

BMI 30–34.9 kg/m² with a weight-related condition and referral to a specialist weight management service.

Mounjaro on the NHS

NICE has approved Tirzepatide for obesity management, but rollout may vary by area. Access often requires:

Meeting obesity criteria
Participation in a structured weight management programme
Lifestyle support alongside medication
Review of response and safety

Even if you meet criteria, many people cannot get these from a GP directly. In many areas, access is through:

Tier 3 / specialist weight management services
Community obesity pathways
Hospital-led services

Waiting lists can be long.

Ethnicity Adjustments

Lower BMI thresholds may apply for some ethnic minority groups because metabolic risk can occur at lower BMI levels.

Common Side Effects

The most common side effects are gastrointestinal:

Nausea Constipation Diarrhoea Reduced appetite

These usually improve during dose titration.

Rare but serious risks

⚠️	Pancreatitis
⚠️	Gallbladder disease

They are contraindicated in people with:

Personal or family history of medullary thyroid carcinoma
MEN2 syndrome

Do GLP-1 Injections Replace Lifestyle Changes?

No.

The strongest outcomes in clinical trials occurred alongside:

✔	Calorie control
✔	Increased protein intake
✔	Behavioural support
✔	Physical activity

Long-term weight maintenance requires sustained lifestyle intervention.

The chronic disease model of obesity is supported by the World Health Organisation, which recognises obesity as a complex metabolic condition requiring multi-factorial treatment. WHO

Support from trusted clinicians matters because sustainable weight loss also depends on building habits around nutrition, movement, sleep, stress management, and understanding the behaviours that contributed to weight gain in the first place.

Many people lose weight initially, but maintaining progress is easier when there is ongoing guidance, accountability, and a plan for life during and after treatment. The most successful outcomes often come when GLP-1s are used as part of a wider support system, not as a standalone solution.

Why Does My Appetite Change?

Appetite changes on GLP-1 medications such as Wegovy and Mounjaro happen because these medicines act on hormone pathways in the brain and gut that regulate hunger and fullness. They enhance signals that tell the brain you are satisfied after eating, while also slowing stomach emptying, which means food stays in the stomach longer and you feel fuller for more time.

At the same time, they reduce “hunger signalling” in areas of the brain involved in reward and cravings, which is why many people describe a reduction in “food noise.” These combined effects naturally lead to a lower appetite and reduced interest in food, rather than requiring conscious restriction or willpower.

Why Do Some People Experience Hair Loss or Fatigue?

Rapid weight loss, regardless of method, can trigger:

Telogen effluvium (temporary hair shedding) Iron deficiency Reduced protein intake Low B12

This highlights the importance of nutritional monitoring during treatment.

Frequently Asked Questions

How quickly do GLP-1 injections work?

Appetite suppression often begins within weeks. Clinically meaningful weight loss typically develops over 3-6 months.

Do you regain weight after stopping Mounjaro or Wegovy?

Data from extension studies suggest weight regain can occur if treatment and lifestyle support are discontinued.

Which causes more weight loss Mounjaro or Wegovy?

Clinical trial data suggests higher average reductions with Mounjaro, but individual variation is significant.

Are GLP-1 injections safe long-term?

Long-term cardiovascular outcome trials in diabetes populations suggest safety, but ongoing monitoring remains essential.

Conclusion

GLP-1 weight loss injections such as Wegovy (Semaglutide) and Mounjaro (Tirzepatide) represent a major advancement in obesity treatment.

They work by modifying appetite signalling and metabolic pathways, not by "boosting metabolism," but by helping regulate energy intake.

However, they are medical therapies, not lifestyle replacements.

The best outcomes occur when medication is combined with:

Nutritional optimisation Muscle preservation strategies Behavioural change Clinical supervision

How Can TribElle Help?

We're not here to sell you a dream. We're here to back you up with:

Expert consultations with female-led clinicians who understand the mental and hormonal load.

Safe prescriptions with support every step of the way - including choosing between Wegovy and Mounjaro.

Ongoing check-ins, supplement guidance, and reorder options that don't make you jump through hoops.

And if you're still unsure? That's okay. Your body, your timeline.

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