Prescription Fat Loss vs Over-The-Counter Weight Loss Medicines: what’s actually evidence-based?
Search for “best weight loss pill” and you’ll see everything from prescription fat loss injections to “natural” diet pills and gummies that claim to “melt fat while you sleep”.
The problem:
not all weight loss medicines are regulated in the same way, and the quality of evidence behind them varies a lot.
In the UK there’s a big difference between:
Prescription-only medicines (POM)
Over-the-counter (OTC) / pharmacy medicines
Food supplements marketed for weight loss
Understanding that difference is key if you want evidence-based and safe support for weight loss.
How medicines are classified in the UK
Very simplified, UK medicines fall into three broad groups:
Prescription-only medicines (POM)
Must be prescribed by a doctor or other qualified prescriber
Supplied by a registered pharmacy
Assessed by regulators (like the MHRA) for quality, safety and effectiveness
Pharmacy medicines (P / “OTC”)
Can be bought without a prescription, but only under the supervision of a pharmacist
Still licensed medicines with evidence behind them
The pack size, dose and who they’re suitable for are tightly controlled
General sale medicines & food supplements
Things like paracetamol in a supermarket, vitamins, herbal products and “slimming supplements”
Supplements are usually regulated as foods, not medicines
They don’t have to prove they cause weight loss in clinical trials before being sold
So while “diet meds”, “diet pills” and “fat burners” all sound similar, they actually sit in very different regulatory buckets, with very different levels of evidence and oversight.
Orlistat: one medicine, two categories (Xenical vs Alli)
Orlistat is a good example of how the same active ingredient can exist in both prescription and OTC form.
Orlistat 120 mg (brand example: Xenical)
Licensed as a prescription-only medicine in the UK and Europe
Usually prescribed for adults with obesity or overweight plus risk factors, alongside a reduced-calorie, lower-fat diet
Orlistat 60 mg (brand example: Alli)
Licensed as a non-prescription (OTC) medicine for adults with BMI ≥ 28 kg/m², under pharmacist supervision
Lower dose, smaller pack sizes, strict advice on how long it can be used without review
Both forms had to show clinical trial evidence that they support weight loss when combined with a suitable diet. But:
The prescription strength is reserved for higher-risk patients under closer medical supervision
The OTC strength is designed for a narrower group, with pharmacists acting as gatekeepers and clear limits on self-treatment
This is very different from unregulated “fat burners” sold online that haven’t had to prove they work – or even that the ingredients on the label are accurate.
Prescription vs OTC vs supplements: key differences
Here’s a simplified comparison of prescription weight loss medicines, OTC medicines and supplements.
| Category | Who decides / prescribes? | Where you get it | Evidence for weight loss | Main risks | Typical examples* |
|---|---|---|---|---|---|
| Prescription-only medicines (POM) | Doctor or other independent prescriber after medical assessment | Registered UK pharmacy with a valid prescription | Must show clear benefit in clinical trials for weight management, with ongoing safety monitoring | Side effects, interactions, not suitable for some conditions; serious risk if misused or sourced illegally | Orlistat 120 mg, certain prescription weight-management injections or tablets |
| Pharmacy (OTC) medicines | Pharmacist checks suitability; no prescription needed but screening questions asked | Community or online pharmacy (with pharmacist oversight) | Must still show evidence of benefit in trials, but doses and packaging are restricted | GI side effects (for orlistat), inappropriate use if BMI is too low or diet is very high-fat | Orlistat 60 mg (Alli) with a reduced-calorie, lower-fat diet |
| Supplements / “fat burners” | Self-selected by the customer; no medical assessment required | Online stores, health shops, influencers, gyms, sometimes international sites | Often little or no robust clinical evidence of meaningful weight loss; many rely on small or poor-quality studies | Variable quality control; risk of contamination, unknown ingredient doses; some cases of liver injury and other harms reported in association with unregulated slimming products | Caffeine “burners”, green tea extract, raspberry ketones, “detox” teas, herbal blends |
*Brand names here are for educational illustration only, not recommendations.
Why prescription medicines are held to a higher standard
To be approved as a prescription weight loss medicine, a product must usually show in trials that:
People taking it with lifestyle changes lose significantly more weight than those making lifestyle changes alone
The benefits outweigh the risks for clearly defined patient groups
Safety is monitored over time, with regulators able to issue warnings or restrict use as new data appears
Even then, the right dose, duration and follow-up depend on your individual medical history. That’s why these medicines stay in the POM category.
By contrast, most “diet supplements” are not required to prove they produce clinically meaningful weight loss. Many have:
Small weight changes at best in short-term studies
Or no high-quality evidence at all
Or marketing claims that go way beyond what the data supports
Why phentermine and Qsymia aren’t used in the UK
If you read US-based websites, you’ll often see drugs like phentermine or phentermine/topiramate (Qsymia) listed as the “best weight loss drug” or “most effective weight loss medication”.
In the UK and Europe, the story is very different.
Phentermine
Phentermine is a stimulant-type appetite suppressant, structurally similar to amphetamine
Its marketing licence in the UK was withdrawn due to safety concerns, including high blood pressure, heart rhythm problems, stroke and dependence
Some sources note that it can still exist under tightly controlled “specials” or historic licences, but it is not widely prescribed for obesity and many clinicians avoid it because of safety and misuse concerns
Qsymia (phentermine/topiramate)
Phentermine/topiramate extended-release (Qsymia in the US) combines phentermine with an anti-seizure medicine
While licensed in the United States, the European Medicines Agency refused authorisation over concerns about long-term safety, including cardiovascular and mental health risks
As a result, Qsymia isn’t licensed or available in the UK for obesity management
So if a website or influencer in the UK is offering to sell you phentermine or Qsymia without a proper prescription and regulatory framework, that’s a major red flag.
Why some OTC products look “stronger” than they are
You’ll also see claims like “pharmacy-strength weight loss without a prescription” or “works like prescription appetite suppressants”.
Often, that marketing:
Piggybacks on the reputation of genuine prescription medicines
But offers a supplement with caffeine, plant extracts or unproven ingredients instead
Or uses wording that makes it sound like a licensed medicine when it’s not
If it’s not clearly labelled as a licensed medicine with an MHRA product licence number, and doesn’t list a known active ingredient and dose (e.g. “orlistat 60 mg”), it’s likely not in the same evidence category as a real weight loss medicine.
The real safety issue: illegal online “prescription” products
One of the biggest current concerns for UK regulators is fake or illegally supplied weight loss medicines:
The MHRA has warned repeatedly about unsafe fake weight loss pens and unlicensed products being sold online and via social media, often copying the look of genuine injection pens
Recent raids have seized hundreds of thousands of pounds’ worth of counterfeit pens and raw chemicals, including products containing substances that aren’t licensed in the UK at all
Health agencies and professional bodies stress that weight loss medications should only be obtained from registered providers using a prescription, and never from beauty salons, social media sellers or friends
Strong message – and this aligns with MHRA guidance:
Never buy prescription medicines without a prescription, and never from unregulated sources.
Doing so can expose you to:
Wrong dose (too high or too low)
Completely different substances from what’s on the label
Contamination, poor storage and no medical monitoring
Serious side effects, including the risk of hospitalisation or worse
How to think about “evidence-based” in practice
When you’re comparing prescription fat loss medicines, OTC diet meds and supplements, you can ask:
Is this actually licensed as a medicine in the UK?
Check for a product licence and recognised active ingredient (e.g. orlistat).
What evidence is there for weight loss?
Has it been studied in proper clinical trials against placebo or lifestyle alone?
Who is it suitable for?
Most genuine weight management medicines have BMI and health criteria and are not for everyone.
Who is monitoring me?
Is there a doctor, pharmacist or regulated clinic involved? Or just a checkout page?
What are the risks and trade-offs?
No medicine is risk-free. The key question is whether, for you, benefits outweigh risks – and that’s a medical judgement.
When supplements can still have a place
This doesn’t mean all supplements are useless. Some nutrients and ingredients can support:
General health (e.g. vitamin D if you’re deficient)
Appetite regulation or energy if part of a structured, supervised plan
Sleep and stress management, which indirectly affect weight
But for meaningful, sustained weight loss, the evidence is much stronger for:
Lifestyle interventions (nutrition, movement, behaviour change)
Prescription weight-management medicines in people who meet criteria, under supervision
Surgery or specialist procedures in selected patients
Supplements should be seen as a possible add-on, not a substitute for medically sound obesity treatment.
Talk to a doctor, not just to Google
If you’re wondering whether medical weight management is right for you, the safest next step isn’t to order “the strongest pill” online.
It’s to talk to a doctor or qualified prescriber who can:
Look at your weight history, health conditions and medications
Explain what evidence-based options actually exist for someone like you
Help you balance effectiveness, safety and your own preferences
Decide whether a prescription medicine, an OTC option like orlistat 60 mg, or a lifestyle-only approach is appropriate – or whether you should be referred for specialist care
