Do weight loss pills work?
Only licensed ones, by very different amounts. No herbal pill has good proof of a loss that matters. Orlistat adds a little. The pill that did most needs a prescription.
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Do weight loss pills work?
Some do, and they’re licensed medicines. The best known is orlistat. It stops your body absorbing some of the fat you eat.
In 11 trials of a year or more, people on orlistat lost 2.7 kg more than people on a dummy pill. Both groups also changed how they ate and moved.
The pill that did most in trials needs a prescription. That pill is the Wegovy tablet (semaglutide).
It’s an appetite suppressant: it helps you feel fuller for longer. In its main trial, people lost 13.6% of their weight over 64 weeks, against 2.2% on a dummy pill.
Sources: Cochrane: orlistat; OASIS 4 trial
For someone who weighs 100 kg, that’s about 3 kg against 11 kg. The trials differ in length and people, so treat it as a rough guide, not a race.
And the fat burners on shop shelves? A review of 54 trials found not enough evidence to recommend any herbal pill.
Your move: check your BMI (weight-for-height score) first. Every licensed pill has a BMI rule.
What are the best over the counter weight loss pills?
Orlistat 60 mg, sold as alli, is the licensed one. The leaflet says it’s for adults 18 and over with a BMI of 28 or above, alongside a lower-calorie, lower-fat diet.
How much does it add? The leaflet’s own words: for every 2 kg you lose from dieting alone, alli might help you lose up to 1 kg more.
Using alli from a pharmacy
- 1
Check you can take it
18 or over, BMI 28 or above, not pregnant or breastfeeding. Ask the pharmacist first if you have liver or kidney disease.
- 2
With each main meal
One capsule, up to three times a day. No fat in the meal? Skip it.
- 3
Vitamins at bedtime
A daily multivitamin with vitamins A, D, E and K.
- 4
Check at 12 weeks
No weight lost? See a pharmacist or doctor. Stop by six months.
Sources: alli leaflet; NHS
The catch shows up in the toilet. Fatty or oily poo, needing to go urgently and wind with oily spotting are common side effects. They’re less likely if you avoid very fatty meals.
On the combined pill? Severe diarrhoea, or diarrhoea for more than 24 hours, may stop it protecting you. Use condoms, a diaphragm or a cap as well.
Orlistat can mix badly with warfarin, levothyroxine and some other medicines. Tell your pharmacist everything you take.
Which weight loss pills need a prescription?
| Pill | How you get it | Who it’s for |
|---|---|---|
| alli (orlistat 60 mg) | From a pharmacy, no prescription. | Adults 18 or over with a BMI of 28 or above. |
| Xenical (orlistat) | Prescription. | BMI of 30 or more, or 28 or more with a condition like type 2 diabetes. |
| Wegovy tablet (semaglutide) | Prescription only. | Adults with a BMI of 30 or more, or 27 to 30 with a weight-related health problem. |
Sources: alli leaflet; NHS; MHRA
On orlistat for 3 months and not lost 5% of your weight? The NHS doesn’t advise carrying on. Your doctor may talk to you about other options.
The Wegovy tablet was approved in the UK in June 2026. When it was approved, it wasn’t available on the NHS.
You swallow it whole with a sip of water after at least 8 hours without food. Then wait 30 minutes before you eat or drink.
Feeling or being sick, diarrhoea and constipation are the most common side effects. In the trial, 74% had a gut side effect, against 42% on a dummy pill.
The NHS also uses injections such as semaglutide and tirzepatide. See Wegovy blood tests and Mounjaro weight loss by week.
Never stop or change a prescribed medicine without your prescriber.
Do fat burners and carb blockers work?
Not by enough to matter. A 2020 review counted 2.5 kg or more as a real difference. None of the common fat burners got there with good proof.
| Pill | What trials found | The catch |
|---|---|---|
| White kidney bean extract (carb blocker) | Beat a dummy pill. | Not by enough to matter. |
| Green coffee bean extract | 2.47 kg more lost, in 3 trials. | All 3 trials were poorly run, so the result may not hold. |
| Green tea extract | No effect seen. | In the review of 54 herbal trials. |
| Garcinia cambogia | No effect seen. | In the same review. |
Sources: Maunder 2020; Onakpoya 2011
Most herbal trials ran for 12 weeks or less. So nobody knows much about taking them for longer.
Want a pill for belly fat, or vitamins to boost your metabolism? We found no review showing either works. The one vitamin advice we found is for orlistat users: a multivitamin at bedtime.
For chromium, glucomannan and more, see natural weight loss supplements.
Are diet pills bought online safe?
Not always. The UK medicines regulator (MHRA) says illegal weight loss products may be fake, contaminated or wrongly dosed. Some hold strong drugs not listed on the pack.
They can cause heart problems, effects on the mind and dangerously low blood sugar. The MHRA’s rule for medicines like Wegovy: if you’re offered one without a prescription, walk away.
Some sold as fat burners are far worse. DNP is a highly toxic industrial chemical sold illegally as a diet pill. It has caused a significant number of deaths in the UK.
Checks before you buy weight loss pills
- Buy medicines only from a registered UK pharmacy. Check it on the General Pharmaceutical Council website, or the Pharmaceutical Society of Northern Ireland (PSNI) one.
- Never buy a prescription-only medicine without a prescription.
- Skip products on social media that promise quick fixes or amazing results, or have unusually low prices.
- Avoid anything labelled DNP or 2,4-dinitrophenol.
- Talk to your GP or a pharmacist before you start any weight loss medicine.
- Report suspicious websites or products to the MHRA Yellow Card scheme.
Sources: MHRA; GOV.UK: DNP
Want help that doesn’t come in a packet? See weight loss programmes and calories to lose weight.
When to speak to a GP
The NHS says losing 5 to 10% of your body weight can improve your health. A GP can help you get there. Speak to one if:
- Your BMI is overweight or obese and you want help to lose weight, or your waist is more than half your height.
- Your BMI is overweight or obese and you have pre-diabetes, diabetes, high blood pressure or high cholesterol.
- Your BMI is overweight or obese and you have joint pain, heart or circulation disease, or sleep apnoea (breathing that stops and starts in sleep).
- You have, or think you may have, an eating disorder, such as binge eating.
- You’ve lost no weight after 12 weeks on alli. The leaflet says to see your doctor or pharmacist.
Taking orlistat and think you have a serious side effect? Call NHS 111.
Signs include severe tummy or back pain that won’t go away, or bleeding from your bottom. So do a swollen throat or tongue, a raised, itchy rash, or trouble breathing.