Three weight-management medicines were registered in Singapore before the GLP-1 class arrived, and all three are still registered today: phentermine since 1988, orlistat since 2005, naltrexone-bupropion since 2022. They were not withdrawn and they were not superseded on paper. What changed is the size of the effect: set the trials side by side below and the averages differ by roughly a factor of three. One thing did not change at all. This article sets out what is registered, what each trial found, and where the older evidence base is still the stronger one.
Availability in Singapore. This medicine is registered with Singapore's Health Sciences Authority, and its registered indication includes weight management. Which medicine suits a particular person, if any does, is decided by the doctor who assesses them — we do not promote a named product.
What is still registered here
| Molecule | Registered since | Classification |
|---|---|---|
| phentermine | 1988, and 1990–1991 for a second brand | Prescription Only |
| orlistat | 2005, and 2017 for a second brand | Pharmacy Only |
| naltrexone with bupropion | 3 January 2022 | Prescription Only |
Source: the register (HSA).
Note the middle row. Orlistat is the one weight-management medicine registered in Singapore that is not Prescription Only — it is Pharmacy Only, supplied by or under a pharmacist's supervision. That does not make it unregulated, and it does make one common sentence wrong: it is not true that every weight-loss medicine here is prescription-only. It is true of the GLP-1 class.
Two labels with a limit written into them
Most registered indications say who a medicine is for. Two of these say when to stop.
Phentermine is described in its indication as an anorectic agent indicated in the management of obesity as a short-term adjunct in a medically monitored comprehensive regimen of weight reduction, in patients with a BMI of 30 or greater — and may be initiated in overweight patients with a BMI of 25 to 29.9 where that carries increased morbidity risk (NDF). "Short-term" is inside the licence, not a footnote to it.
Orlistat's indication covers obese patients with a BMI of 30 or above, or overweight patients with a BMI of 28 or above with associated risk factors, in conjunction with a mildly hypocaloric diet — and states that treatment should be discontinued after 12 weeks if the patient has not lost at least 5% of initial body weight (NDF).
Naltrexone-bupropion's indication is written like the newer ones: an adjunct to a reduced-calorie diet and increased physical activity for chronic weight management in adults with a BMI of 30 or greater, or 27 or greater with at least one weight-related comorbid condition (NDF).
The trials, side by side
| Trial | Medicine | Duration | Result |
|---|---|---|---|
| XENDOS | orlistat | 4 years, n=3,305 | −5.8 kg vs −3.0 kg |
| COR-I | naltrexone-bupropion | 56 weeks, n=1,742 | −6.1% vs −1.3% |
| STEP 1 | semaglutide 2.4 mg | 68 weeks, n=1,961 | −14.9% vs −2.4% |
| SURMOUNT-1 | tirzepatide 15 mg | 72 weeks, n=2,539 | −20.9% vs −3.1% |
Sources: Torgerson 2004, Greenway 2010, Wilding 2021, Jastreboff 2022. Individual results vary and are not guaranteed.
These are four separate trials in different populations over different periods, so the comparison is indicative rather than head-to-head — the same caution that applies to any cross-trial reading. A 2026 network meta-analysis that pooled 262 trials and modelled them against a common comparator found the same ranking: at one year against lifestyle modification alone, −14.9% for tirzepatide, −9.8% for subcutaneous semaglutide, and −8.1% for phentermine-topiramate (Nong 2026).
One caveat on that last figure: phentermine-topiramate is a different combination product, and it is not registered in Singapore. It is not the phentermine on the table above, and the number should not be read onto it.
Where the older evidence still leads
Duration. XENDOS ran for four years. The GLP-1 obesity trials above ran 56 to 72 weeks. Four years of randomised follow-up in 3,305 people is a kind of evidence the newer class does not yet have in obesity, and it is worth more than the headline percentage suggests.
A hard endpoint. XENDOS did not only weigh people. Cumulative incidence of type 2 diabetes over those four years was 6.2% with orlistat against 9.0% with placebo — a relative risk reduction of 37.3% (Torgerson 2004). The trial's own exploratory analysis attributed the effect to the subgroup with impaired glucose tolerance at baseline.
A different mechanism. Orlistat works in the gut, reducing how much dietary fat is absorbed. It does not act on appetite in the brain, which is what the GLP-1 class does. For someone in whom the newer medicines are unsuitable, it is a different tool rather than a weaker version of the same one.
Tolerability is not a footnote. The same 2026 analysis found discontinuation because of adverse events among the highest with naltrexone-bupropion, and fatigue risk most increased with it, at a risk ratio of 8.9 (Nong 2026). In COR-I, half the participants completed the 56 weeks (Greenway 2010). A medicine someone can stay on beats one they stop, whichever era it came from.
What none of them changed
Every trial on this page tested a medicine alongside diet and physical activity, and every one of these labels says adjunct.
That was true of the 1988 registration and it is true of the 2025 ones. The medicine has never been the whole intervention in any era, and no result in the table above was produced by a medicine on its own.
The second thing that has not changed is what the trials measured. They weighed people. None of these medicines is doing anything to ensure that what comes off is fat rather than muscle — that is decided by protein intake, resistance training and the rate of loss, and it is the difference between being lighter and being leaner. We cover it in how much muscle you lose on GLP-1 medication.
The wider landscape, including surgery and the non-medical options, is compared in the weight-loss options guide, and surgery specifically in GLP-1 vs bariatric surgery.
Whether any of this is suitable for you is a decision for a doctor who has assessed you, and nothing here is a reason to stop or change a medicine you were prescribed.
Common questions
Is phentermine still available in Singapore?
Yes. Phentermine products have been registered here since 1988 and 1990–1991 and are classified Prescription Only (HSA register). Its registered indication describes it as a short-term adjunct in the management of obesity (NDF).
How much less effective are the older medicines?
Setting the trials side by side, the averages differ by roughly a factor of three. XENDOS reported 5.8 kg against 3.0 kg with placebo over four years for orlistat (Torgerson 2004); COR-I reported −6.1% against −1.3% over 56 weeks for naltrexone-bupropion (Greenway 2010); STEP 1 reported −14.9% and SURMOUNT-1 up to −20.9% (Wilding 2021, Jastreboff 2022). These are separate trials in different populations, so the comparison is indicative rather than head-to-head. Individual results vary.
Do I need a prescription for orlistat?
Orlistat is classified Pharmacy Only rather than Prescription Only (HSA register) — supplied by or under the supervision of a pharmacist. It is the one weight-management medicine registered here that sits outside the prescription-only class, which is why a blanket statement that every weight-loss medicine in Singapore is prescription-only is not accurate.
Is there anything the older medicines do better?
Duration of evidence. XENDOS ran for four years and reported a hard clinical endpoint — type 2 diabetes incidence of 6.2% against 9.0%, a 37.3% relative risk reduction (Torgerson 2004). Most of the GLP-1 obesity trials ran 56 to 72 weeks.
Should I switch from one to the other?
That is a decision for the doctor who prescribed what you are taking, and it turns on how you have responded, what you tolerate and your medical history. Nothing on this page is a reason to stop or change a medicine.