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GLP-1 medicines and weight: what they are, and what they are not

Plain information for patients in Singapore about a group of prescription medicines used in the treatment of obesity, where they fit among the other options, and their risks.

What GLP-1 receptor agonists are

GLP-1 stands for glucagon-like peptide-1, a hormone the gut releases after a meal. It helps signal fullness, slows how quickly the stomach empties, and prompts the pancreas to release insulin when blood sugar rises.

GLP-1 receptor agonists are medicines that mimic that hormone. They were developed for type 2 diabetes, and some were later registered for the treatment of obesity as well. A newer group acts on a second gut hormone, GIP, in addition to GLP-1. For weight management they are given as a weekly or daily injection under the skin. A tablet form of semaglutide also exists, but in Singapore it is registered for type 2 diabetes only, not for weight loss. The active ingredients registered here include liraglutide, semaglutide and tirzepatide.

All of them are prescription-only medicines. That is a legal classification: they may be supplied only on a doctor's prescription, after a consultation, because the decision to use them depends on a clinical assessment of the individual patient.

Who they may be considered for

In Singapore, weight is judged against Asian BMI cut-offs, which are lower than the international ones because health risks rise at a lower BMI in Asian populations. A BMI of 23 to 27.4 carries a moderate risk of weight-related health problems, and 27.5 or above a high risk.

Medicine is reserved for a narrower group. The Ministry of Health's clinical practice guidelines on obesity say weight-loss medicines may be considered for adults with:

• a BMI of 30 or above, or
• a BMI of 27.5 to 29.9 together with a weight-related condition, such as high blood pressure, type 2 diabetes or high cholesterol.

The product information registered in Singapore for these medicines uses similar thresholds: a BMI of 30 or above, or 27 or above with at least one weight-related condition, such as high blood pressure, high cholesterol, obstructive sleep apnoea, heart disease, prediabetes or type 2 diabetes. In every case the medicine is used alongside a reduced-calorie diet and more physical activity, not instead of them.

Meeting a threshold does not mean a medicine is right for you. The decision depends on your overall health, your other medicines, what you have already tried and the risks below, and it can only be made by a doctor who has assessed you.

How they affect appetite and weight

People describe the main effect as feeling full sooner and staying full for longer, and thinking about food less. Eating less follows from that, rather than from willpower. Slower stomach emptying contributes, which is also why the common side effects are digestive.

Most are given as an injection into the fat just under the skin of the abdomen, thigh or upper arm, using a pen-style injector with a very short, fine needle rather than a conventional syringe. Patients are shown how to use the pen before they start.

The effect lasts while the medicine is being taken. These medicines treat the condition; they do not cure it.

What the evidence shows, and what it does not

In randomised clinical trials, adults with obesity who took these medicines alongside diet and activity changes lost substantially more weight on average than those given a dummy injection with the same diet and activity advice. Trials of some of these medicines have also reported improvements in blood sugar control and blood pressure, and one large trial in patients with existing cardiovascular disease reported a reduction in cardiovascular events.

What the evidence does not show is a predictable result for any individual. Average results in a trial are not a forecast for one person: response varies widely, some people respond very little, and some cannot tolerate the medicine. Trial participants also received structured dietary and activity support throughout, which is part of why they did well.

Where they sit among the other options

Medicine is one part of treating obesity, not the whole of it. The recognised options are:

• Diet, physical activity and sleep. The foundation of every approach, including treatment with medicine, and the part that determines whether any gains last.
• Treating contributing conditions and reviewing medicines that promote weight gain, such as some treatments for diabetes, mood disorders and epilepsy.
• Prescription medicines. Besides the GLP-1 group, other classes are registered in Singapore, including appetite suppressants and medicines that reduce fat absorption. They differ in how they are taken, their side effects and who they suit.
• Bariatric and metabolic surgery, for selected patients, usually where obesity is severe or where medical treatment has not achieved enough.

Which of these is appropriate, if any, depends on your BMI, the conditions you have, what you have already tried, and your own preferences after a discussion with your doctor.

Side effects and risks

The common side effects are digestive: nausea, vomiting, diarrhoea, constipation, indigestion and abdominal discomfort. They are usually worst when starting treatment or increasing a dose, and often settle. Dose is normally increased gradually for this reason.

Less common but more serious risks include inflammation of the pancreas, gallstones and gallbladder disease, dehydration and kidney problems following persistent vomiting or diarrhoea, and low blood sugar when the medicine is combined with certain diabetes treatments. Some people lose muscle as well as fat, which is one reason protein intake and physical activity matter during treatment.

Because these medicines slow how quickly the stomach empties, food can remain in the stomach for longer than usual. Tell the doctor or dentist in charge before any procedure that needs sedation or a general anaesthetic, as there is a risk of stomach contents entering the lungs. In people with diabetic eye disease, a rapid improvement in blood sugar can temporarily worsen the eye condition, so eye checks may be needed.

Anyone taking one of these medicines should know what warrants urgent attention. Severe, persistent abdominal pain, particularly with vomiting, needs prompt medical assessment.

Who should not take them

• People who are pregnant, breastfeeding or planning a pregnancy.
• People with a personal or family history of medullary thyroid carcinoma, or with multiple endocrine neoplasia syndrome type 2.

They are used with caution, if at all, in people who have had pancreatitis, and in people with certain other conditions, including significant kidney or liver disease, diabetic eye disease and some eating disorders, where the risks may outweigh the benefits.

This is not a complete list, and the specific cautions differ between medicines. A doctor screens for these before prescribing, and reviews them again as treatment continues.

What happens when treatment stops

Appetite generally returns to what it was, and weight is commonly regained, sometimes most of it. This is expected rather than a sign that the treatment failed, and it is the reason obesity is described as a long-term condition. Anyone considering these medicines should discuss with their doctor what happens afterwards, and over what period they might be used.

Buying them online, and why it is risky

These medicines cannot lawfully be sold to the public in Singapore without a prescription, and the Health Sciences Authority has taken action against unauthorised online sales. Products bought from unlicensed sellers, or compounded versions, may contain the wrong dose, a different substance or nothing active at all, and they come without the assessment and monitoring that make treatment safe. If you are considering treatment, see a doctor.

Sources: Ministry of Health Clinical Practice Guidelines: Obesity (2016), summarised in Singapore Medical Journal 2016;57(6):292-300; Health Promotion Board, BMI and health risk (HealthHub); product information for medicines registered by the Health Sciences Authority, as published in the National Drug Formulary.
Written for patients in Singapore. General health information only: it is not medical advice, not a recommendation for any particular medicine, and not an offer or promotion of any treatment or product. No brand of medicine is recommended or supplied on the basis of this page. Prescription medicines are supplied only on a doctor's prescription following a consultation.
Last reviewed: 24 September 2026.