Weight-loss injections have quickly become part of everyday health conversations.
You may have heard names such as Ozempic, Wegovy, or Mounjaro, seen striking transformation stories online, or wondered why medicines originally developed for diabetes lead to substantial weight loss.
But what actually happens inside the body when someone takes one?
The answer involves more than simply eating less. These treatments influence signals between the gut, brain, and pancreas that regulate appetite, blood sugar, and fullness after eating.
Key Takeaways
- Semaglutide and tirzepatide act on hormonal pathways involved in appetite and blood sugar regulation.
- They can reduce hunger, increase feelings of fullness, and influence how much people eat.
- They also affect insulin and glucagon, which is why this class first became important in the treatment of diabetes.
- Clinical trials have shown substantial average weight loss, but individual responses vary.
- Weight loss includes both fat and some lean tissue, making good nutrition and resistance exercise important considerations.
- Weight regain is common after treatment stops.
- These are prescription medicines with potential side effects and are not appropriate for everyone.
Are Ozempic, Wegovy and Mounjaro the Same Thing?
The brand name matters because related medicines have different approved uses.
Semaglutide is the active ingredient in both Ozempic and Wegovy.
In Singapore:
- Ozempic is registered for the treatment of type 2 diabetes.
- Wegovy is registered for chronic weight management in eligible adults and adolescents, alongside cardiovascular risk reduction in certain adults.
- Mounjaro, which contains tirzepatide, is registered for both type 2 diabetes and weight management in eligible adults.
Although these treatments share underlying mechanisms, they are not interchangeable.
What Are GLP-1 Medicines?
GLP-1 is a hormone your body naturally releases after eating.
Its full name is glucagon-like peptide-1.
GLP-1 coordinates post-meal responses by:
- Signalling fullness to the brain
- Stimulating insulin release when blood glucose is elevated
- Reducing glucagon secretion
- Slowing gastric emptying
Natural GLP-1 degrades rapidly in the bloodstream.
Medicines such as semaglutide activate the GLP-1 receptor for extended periods, allowing for weekly administration.
Tirzepatide activates both GLP-1 and GIP receptors. Glucose-dependent insulinotropic polypeptide (GIP) is another gut hormone involved in metabolic regulation.
Step 1: They Alter Hunger Signals
GLP-1 receptor agonists act primarily on brain pathways regulating appetite and intake.
As a result, patients taking semaglutide or tirzepatide often experience:
- Reduced hunger and earlier satiety
- Smaller portion sizes and prolonged fullness
Many report less “food noise” (persistent food thoughts), and research continues into how these therapies modulate reward signals and eating behaviour.
Step 2: They Slow Gastric Emptying
By slowing gastric emptying, GLP-1 receptor activation enhances early satiety.
This effect is most pronounced during initial treatment, though overall digestive transit continues and the intensity typically moderates over time.
Step 3: They Regulate Blood Sugar
These treatments stimulate glucose-dependent insulin secretion and suppress glucagon after meals.
This glucose-dependent action lowers hypoglycaemia risk compared with older diabetes therapies, though risk increases when combined with insulin or sulfonylureas. These glycemic benefits led to their initial approval for type 2 diabetes.
Weight Loss Outcomes
In the 72-week SURMOUNT-5 trial (751 adults with obesity, without diabetes), maximum tolerated doses produced significant average weight reduction:
| Medicine | Average Weight Reduction |
| Tirzepatide | 20.2% |
| Semaglutide | 13.7% |
Trial averages, achieved alongside lifestyle counselling, do not guarantee individual outcomes.
Retatrutide Status
Retatrutide is an investigational triple agonist (GIP, GLP-1, glucagon receptors).
Despite substantial Phase 3 weight loss findings in 2026, it remains unapproved as of August 2026. Unregulated online products claiming to contain retatrutide pose serious health risks.
Is All the Weight Lost Body Fat?
Weight loss from incretin therapies includes reductions in both fat mass and lean body mass.
Lean mass encompasses muscle, organs, bone, and fluid; loss of lean mass is not equivalent to skeletal muscle loss alone.
A 2026 meta-analysis indicated that lean mass accounted for approximately:
- 35% of total weight lost with semaglutide
- 25% with tirzepatide
- 27% with liraglutide
These proportions mirror lean mass changes seen in calorie-restricted lifestyle interventions. Incorporating resistance training and adequate dietary protein helps preserve functional muscle mass during treatment.
What Happens if You Stop Taking the Medicine?
Weight regain is common after treatment stops.
In the STEP 1 trial extension, participants who discontinued semaglutide regained roughly two-thirds of lost weight over the subsequent year. Withdrawal trials for tirzepatide demonstrate similar trends.
Because GLP-1 therapies modify underlying physiological signals, appetite and metabolic pathways return to baseline following treatment cessation. For many patients, obesity management represents a long-term treatment strategy rather than a temporary intervention.
What Side Effects Can Occur?
Gastrointestinal side effects are the most common.
Common side effects include nausea, vomiting, diarrhoea, constipation, and abdominal pain. Symptoms are usually most pronounced during dose escalation.
Less common but serious risks include pancreatitis, gallbladder disease, and severe dehydration. Urgent medical review is required for persistent severe abdominal pain or signs of severe allergic reactions.
Are These Medicines Right for Anyone Who Wants to Lose Weight?
GLP-1 receptor agonists are prescription medicines intended for patients meeting specific clinical criteria.
In Singapore, Wegovy and Mounjaro are indicated for chronic weight management alongside dietary reduction and physical activity in individuals meeting defined BMI thresholds and comorbidity criteria.
Obesity prevalence in Singapore rose from 10.5% in 2019–2020 to 12.7% in 2023–2024, highlighting the burden of metabolic disease. Medical management addresses complex biological factors including genetics, appetite dysregulation, and neuroendocrine signalling.
Do These Medicines Replace Diet and Exercise?
Pharmacotherapy complements lifestyle modification rather than replacing it.
Nutritional quality, regular physical activity, and adequate sleep remain essential for cardiovascular health, functional strength, and overall wellness.
Frequently Asked Questions
Are Ozempic and Wegovy the same medicine?
Both contain semaglutide, but they are registered for different uses and prescribed at different doses. Ozempic treats type 2 diabetes, while Wegovy is approved for weight management.
Why do GLP-1 medicines reduce appetite?
They alter hormone signals between the gut and brain. This affects hunger, fullness, and overall food intake.
Why can these medicines cause nausea?
These medicines slow stomach emptying and affect brain areas linked to appetite and nausea. Symptoms are most noticeable when starting treatment or raising the dose.
Is all the weight lost body fat?
No. Weight loss includes both fat and lean mass. Good nutrition and resistance exercise help preserve muscle strength.
Will the weight return if treatment stops?
Weight regain is common after stopping treatment. For this reason, these medicines are often used as longer-term therapies.
Is retatrutide available now?
No. As of August 2026, retatrutide is still under investigation and not approved for public use.
The Bottom Line
GLP-1-based weight-management medicines work through several connected systems.
GLP-1 medicines alter central and peripheral signals governing hunger, satiety, gastric motility, and insulin secretion. While highly effective for weight reduction, treatment requires clinical evaluation, ongoing management, and integration with health-promoting lifestyle habits.
Editorial Note
This article is intended for general information and should not replace personalised medical advice. Semaglutide, tirzepatide, and other prescription weight-management medicines have specific indications, contraindications, dosing requirements, and potential adverse effects. Eligibility and treatment should be assessed by a qualified healthcare professional. Do not obtain prescription weight-loss medicines or investigational products from unverified or unauthorised sources.
Sources
Health Sciences Authority. “Listing of Approvals – Wegovy.” Health Sciences Authority, 2026. https://www.hsa.gov.sg/therapeutic-products/registration-of-therapeutic-products/tools-and-resources/listing-of-approvals-and-post-registration-actions/listing-of-approvals/
Health Sciences Authority. “New Drug Approvals – Mounjaro.” Health Sciences Authority, 2025. https://www.hsa.gov.sg/announcements/new-drug-approvals—october-2025/
Health Sciences Authority. “Ozempic Summary Report of Benefit-Risk Assessment.” Health Sciences Authority, 2021.
https://www.hsa.gov.sg/therapeutic-products/registration-of-therapeutic-products/tools-and-resources/summary-of-benefit-risk-assessment-reports/archive/
Ministry of Health, Singapore. “National Population Health Survey 2024 Shows Singaporeans Are Adopting Healthier Lifestyles, But Rising Obesity Is a Concern.” Ministry of Health, 2025. https://www.moh.gov.sg/newsroom/national-population-health-survey-2024-shows-singaporeans-are-adopting-healthier-lifestyles—but-rising-obesity-is-a-concern/
Drucker DJ. “Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1.” Cell Metabolism, 2018.
https://pubmed.ncbi.nlm.nih.gov/29617641/
Gabery S, Salinas CG, Paulsen SJ, et al. “Semaglutide Lowers Body Weight in Rodents via Distributed Neural Pathways.” JCI Insight, 2020.
https://insight.jci.org/articles/view/133429
van Bloemendaal L, IJzerman RG, Ten Kulve JS, et al. “GLP-1 Receptor Activation Modulates Appetite- and Reward-Related Brain Areas in Humans.” Diabetes, 2014.
https://pubmed.ncbi.nlm.nih.gov/25071023/
Aronne LJ, Horn DB, le Roux CW, et al. “Tirzepatide as Compared with Semaglutide for the Treatment of Obesity.” New England Journal of Medicine, 2025.
https://pubmed.ncbi.nlm.nih.gov/40353578/
Wilding JPH, Batterham RL, Davies M, et al. “Weight Regain and Cardiometabolic Effects After Withdrawal of Semaglutide: The STEP 1 Trial Extension.” Diabetes, Obesity and Metabolism, 2022. https://pubmed.ncbi.nlm.nih.gov/35441470/
Aronne LJ, Sattar N, Horn DB, et al. “Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial.” JAMA, 2024. https://jamanetwork.com/journals/jama/fullarticle/2812936
Horn DB, Linetzky B, Davies MJ, et al. “Cardiometabolic Parameter Change by Weight Regain on Tirzepatide Withdrawal in Adults With Obesity.” JAMA Internal Medicine, 2026.
https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2841273
Eisa N, Barood O. “Lean Mass Changes With Incretin Therapy Versus Lifestyle Intervention: A Systematic Review and Meta-Analysis of Randomised Controlled Trials.” Diabetes, Obesity and Metabolism, 2026. https://pubmed.ncbi.nlm.nih.gov/41877354/
Eli Lilly and Company. “What to Know About Retatrutide.” Eli Lilly and Company, 2026. https://www.lilly.com/news/stories/what-to-know-about-retatrutide
Health Sciences Authority. “Dubious Weight Loss Products Sold Online Can Harm Your Health.” Health Sciences Authority, 2026.
https://www.hsa.gov.sg/announcements/dubious-weight-loss-products/