Weekly weight-loss injections explained honestly: how they work, the side effects, who should avoid them, and what happens when you stop
Medically reviewed by Dr. Adriene Chee, Medical Director, Verve Clinic, Petaling Jaya. Published 12 October 2026. Last updated 12 October 2026. 7 minute read.
You have probably seen them on social media: a weekly pen, dramatic before-and-after photos, and a friend who swears she lost 10 kg. Then come the questions. Are Mounjaro and Wegovy safe? Will the weight come back? Should I just buy one online? These are fair questions, and you deserve straight answers. You are also far from alone: according to Malaysia’s National Health and Morbidity Survey 2023, 54.4% of adults are overweight or obese.
In short: Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription medicines that reduce appetite. In large trials, people lost an average of about 15% to 21% of their body weight over 68 to 72 weeks. They are considered safe for suitable adults under a doctor’s care, although some people should not use them. Mild nausea or stomach upset can occur in the first weeks and usually settles. Weight can return after stopping if you go back to your old eating habits or binge eating without controlling your diet, so lasting results come from building healthy eating and exercise habits during treatment, rather than depending on the pen long term.
Below, we explain how these “slimming pens” work, what the research shows, the risks worth knowing, and why the way you start and stop matters as much as the medicine itself.
What are Mounjaro and Wegovy?
Both are once-weekly injections, given with a small pen just under the skin of the tummy, thigh or upper arm.
Wegovy contains semaglutide. It copies GLP-1, a hormone your gut releases after eating, which tells your brain you are full and slows how quickly your stomach empties.
Mounjaro contains tirzepatide. It acts on GLP-1 and a second gut hormone, GIP, which is why it is sometimes called a “dual” medicine.
The result for most people is simple: you feel full sooner, think about food less, and find smaller portions easier to stick to. The medicine does not burn fat by itself. It makes eating less feel manageable.
Mounjaro vs Wegovy: what is the difference?
Mounjaro and Wegovy are both weekly weight-loss injections, but they contain different medicines. Mounjaro (tirzepatide) acts on two gut hormones, GIP and GLP-1, while Wegovy (semaglutide) acts on GLP-1 only. In their main trials, average weight loss was up to about 21% with Mounjaro and about 15% with Wegovy, although these were separate studies. Your doctor will advise which, if either, suits you.
| Mounjaro | Wegovy | |
|---|---|---|
| Active ingredient | Tirzepatide | Semaglutide |
| How it works | Acts on two gut hormones (GIP and GLP-1) | Acts on one gut hormone (GLP-1) |
| How it is given | Once-weekly pen, dose increased step by step | Once-weekly pen, dose increased step by step |
| Average weight loss in its main trial | 15% to 20.9% at 72 weeks, depending on dose | About 15% at 68 weeks |
| Most common side effects | Nausea, diarrhoea, vomiting, constipation | Nausea, diarrhoea, vomiting, constipation |
How much weight do people actually lose?
In clinical trials, adults lost an average of about 15% of their body weight on Wegovy over 68 weeks, and up to about 21% on Mounjaro over 72 weeks. These are averages, not promises.
- In the SURMOUNT-1 trial of 2,539 adults, people on tirzepatide lost an average of 15% (5 mg), 19.5% (10 mg) and 20.9% (15 mg) of their body weight over 72 weeks. Those on placebo lost 3.1%.
- In the STEP 1 trial, people on semaglutide 2.4 mg lost an average of about 14.9% over 68 weeks, compared with about 2.4% on placebo.
Everyone in these trials also followed diet and exercise advice. Some people lose much more than average and some lose less. Your own result depends on your starting weight, health, dose and habits.
Are Mounjaro and Wegovy safe? Side effects and risks
For suitable adults, under medical supervision, the short answer is yes, with caveats. Both medicines have been studied in thousands of people, and most side effects are mild to moderate and settle with time.
Common side effects usually appear in the first weeks and after each dose increase:
- Nausea: affected 25% to 33% of people on tirzepatide in weight-management trials (EMA product information)
- Diarrhoea: 19% to 23%
- Vomiting, constipation, tummy discomfort and tiredness
This is why doses start low and rise gradually, usually every four weeks. In SURMOUNT-1, between 4.3% and 7.1% of people on tirzepatide stopped because of side effects, compared with 2.6% on placebo.
Less common but serious risks are the reason these are prescription-only medicines:
- Pancreatitis (inflammation of the pancreas): severe tummy pain that spreads to the back needs urgent review
- Gallbladder problems, such as gallstones, which become more likely with rapid weight loss
- Dehydration and kidney strain, mainly when vomiting or diarrhoea is severe
- Low blood sugar, mainly in people who also take insulin or certain diabetes tablets
- Risks during sedation or anaesthesia, because food stays in the stomach longer. Always tell your doctor or dentist you are on these medicines before any procedure
Who should not use them?
These medicines are not for everyone. Your doctor will usually advise against them, or take extra care, if you:
- Are pregnant, breastfeeding or planning a pregnancy. Tirzepatide should be stopped at least one month before a planned pregnancy
- Have had pancreatitis
- Have a personal or family history of medullary thyroid cancer or a condition called MEN2
- Have a history of an eating disorder
- Only want to lose a few kilograms for an event
Malaysia’s 2023 Clinical Practice Guidelines on obesity suggest considering weight-loss medicine for adults with a BMI of 30 or more, or 27 or more with a weight-related condition such as diabetes, high blood pressure or sleep apnoea, always alongside lifestyle changes.
Will I regain the weight when I stop?
Weight often returns after stopping Mounjaro or Wegovy, especially if you go back to old eating habits or binge eating. In one trial, people who stopped tirzepatide regained 14% of their body weight within a year.
The medicine quiets your appetite only while you take it. When it stops, appetite hormones return to their old levels. Two studies show what happens:
- SURMOUNT-4 (tirzepatide): after 36 weeks of treatment, people had lost an average of 20.9%. Those switched to a placebo regained 14% of their body weight over the next year, while those who stayed on tirzepatide lost a further 5.5%.
- STEP 1 extension (semaglutide): one year after stopping, people had regained about two-thirds of the weight they lost, and most improvements in blood pressure, blood sugar and cholesterol moved back towards where they started.
This does not mean the medicine failed. It reflects what Malaysia’s guidelines now recognise: obesity is a long-term condition, like high blood pressure. Rebound is less likely when you:
- Build eating and activity habits while on treatment, not after
- Reduce the dose gradually with your doctor rather than stopping suddenly
- Keep up strength training and enough protein, because part of the weight lost can be muscle
- Continue regular follow-ups after the pen stops
A note from Dr. Adriene Chee: “A slimming pen helps control your appetite while you reach a healthier weight. Keeping the weight off depends on the habits you build: a healthy diet and regular exercise.”
Is it safe to buy slimming pens online?
No. In Malaysia, semaglutide and tirzepatide are Group B Poisons. They can legally be supplied only by a registered doctor, or by a pharmacist with a valid prescription. In 2025, the Ministry of Health issued warning letters over 48 social media adverts for these pens.
Pens sold online may be fake, wrongly stored, or the wrong strength. You also lose the safety checks: blood tests, a doctor who knows your history, and someone to call when side effects appear.
What does doctor-supervised treatment look like at Verve Clinic?
- Consultation. We discuss your weight history, goals and health, and check your BMI.
- Blood tests. We check blood sugar, cholesterol, liver and kidney function and other markers through our health screening, so we start from a safe baseline.
- Your plan. If a slimming pen suits you, we choose the medicine and starting dose, and explain how to inject and what to expect.
- Regular follow-ups. We track your weight, check side effects and adjust the dose.
- A plan for the long term. We discuss how to maintain your results, including how and when to reduce the dose.
| Quick facts | |
|---|---|
| How it is given | One injection a week, under the skin |
| Dose | Starts low and increases step by step |
| When side effects are most likely | First weeks, and after each dose increase |
| When weight loss is noticeable | Usually within the first few months |
| How long treatment lasts | Planned with your doctor; often long term |
| Follow-ups | Regular weight and symptom checks |
Frequently asked questions
Are Mounjaro and Wegovy safe?
For suitable adults under medical supervision, both have good safety records from large trials. Stomach side effects are common at first. Rare but serious risks include pancreatitis and gallbladder problems, which is why a doctor should assess you first.
Will I regain weight after stopping Mounjaro or Wegovy?
Most people regain some weight if they stop suddenly without lifestyle changes. In one trial, people who stopped tirzepatide regained 14% of their body weight within a year. A gradual, planned approach lowers this risk.
How much weight can I lose on a slimming pen?
In the main trials, average weight loss was about 15% with semaglutide and up to about 21% with tirzepatide over 68 to 72 weeks. Individual results vary.
What are the most common side effects?
Nausea, diarrhoea, vomiting and constipation, mostly in the first weeks and after dose increases. They usually ease with time.
Can I buy Mounjaro or Wegovy online in Malaysia?
No. Both are prescription-only medicines that must be supplied by a doctor or pharmacist. Pens sold online may be fake or unsafe.
Who should not use slimming pens?
People who are pregnant or planning a pregnancy, those with a history of pancreatitis or certain thyroid cancers, and anyone with a history of an eating disorder should avoid them or need specialist advice.
How long do I need to stay on treatment?
It depends on your goals and health. Many people need longer-term treatment or a gradual reduction to keep the weight off.
Is Wegovy the same as Ozempic?
No, although they contain the same medicine. Both contain semaglutide, but Ozempic is approved for type 2 diabetes, while Wegovy is approved for weight management and is used at a higher dose. Your doctor will advise which, if either, suits you.
Do slimming pens cause muscle loss?
They can. When you lose weight quickly, part of what you lose can be muscle as well as fat. Eating enough protein and doing regular strength training help protect your muscle, and your doctor can track your progress at follow-ups.
Thinking about a slimming pen? Start with an honest conversation
If your weight is affecting your health, you do not have to work it out alone. Our doctor will assess whether a slimming pen suits you, run the right blood tests and stay with you through the whole journey. You can read more about our doctor-supervised medical weight loss programme, or WhatsApp us to arrange a consultation and a personalised quote.
About the reviewer
Dr. Adriene Chee is the Medical Director of Verve Clinic in Petaling Jaya, with over 10 years of experience. Her qualifications include an MD from the National University of Malaysia (UKM), a Postgraduate Diploma in Dermatology (Bangkok) and an LCP in Aesthetic Medicine. Meet our team.
This article is general information, not medical advice. Results and suitability vary from person to person. Please discuss the benefits and risks with a qualified doctor before starting any medicine.
