---
title: "Weight loss medicines: Uses & Side Effects | HealthAnswers"
description: "NHS-prescribed weight loss medicines explained — orlistat, semaglutide (Wegovy) and tirzepatide (Mounjaro): who qualifies, how they work, and side effects."
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# Weight loss medicines: Uses & Side Effects | HealthAnswers

Treatments

NHS-prescribed weight loss medicines explained — orlistat, semaglutide (Wegovy) and tirzepatide (Mounjaro): who qualifies, how they work, and side effects.

Written by HealthAnswers editorial team Medically reviewed by [HealthAnswers UK doctor review panel](https://healthanswers.co.uk/our-doctors/) Reviewed 3 July 2026

## Quick answer

What are weight loss medicines used for? NHS weight loss medicines include orlistat (blocks fat absorption), semaglutide/Wegovy and tirzepatide/Mounjaro (reduce appetite via weekly injections). They are for adults with obesity who meet strict criteria — usually within specialist weight management services — not for cosmetic weight loss. All work alongside diet and exercise, not instead of them.

## Who is this page for?

This page is for people in the UK who want a plain-English overview of Weight loss medicines, including when to seek help and what NHS care usually involves. It explains general information, not what is wrong with you personally.

## Who should skip this page?

Skip this page if you need a diagnosis, a prescription, or emergency care. It cannot replace a GP, pharmacist, NHS 111 or 999. If you are in immediate danger, call 999; for urgent advice that is not life-threatening, call 111 in England, Scotland or Wales, or use your GP out-of-hours service in Northern Ireland.

On this page 12 sections

## Key facts about weight loss medicines

- Weight loss medicines require BMI 30+ or 27.5+ with weight-related conditions for NHS prescribing.
- Semaglutide (Wegovy) and tirzepatide (Mounjaro) are weekly injections — not the same as diabetes-only brands without specialist oversight.
- Orlistat is the only weight loss pill routinely available — blocks about one third of dietary fat absorption.
- Typical results — 5 to 15% body weight loss over months when combined with lifestyle changes.
- Stopping medicines often leads to weight regain unless habits change.

## Prescription weight loss medicines — facts not hype

Weight loss medicines have attracted enormous online attention — especially **semaglutide (Wegovy)** and **tirzepatide (Mounjaro)**. They can help some people with **obesity** lose significant weight when combined with diet and activity, but they are **not magic injections**, not suitable for everyone, and on the NHS require meeting **strict clinical criteria** within weight management services.

This guide explains what is actually available in the UK, how each medicine works, and what to expect.

## Who qualifies on the NHS

NICE guidance generally supports weight loss medicines for adults with:

- BMI 30 or above , or

- BMI 27.5 to 29.9 (lower threshold for some ethnic groups) with weight-related comorbidity — type 2 diabetes, high blood pressure, sleep apnoea, etc.

Additional requirements typically include:

- completion of a multicomponent weight management programme (tier 2 or 3)

- self-funded or programme duration criteria met before starting medicines

- agreement to continue diet and activity changes

- monitoring plan in place

Exact pathways vary by NHS integrated care board — availability has been limited by supply and funding.

Weight loss medicines are **not prescribed for cosmetic reasons** in people at healthy weight.

## Orlistat (Xenical, generic)

**How it works:** Blocks pancreatic lipase, an enzyme that breaks down dietary fat. About **one third of fat** in meals passes through undigested.

**Form:** Capsule taken with meals containing fat — up to three daily.

**Expected loss:** Roughly **3 to 5kg** more than lifestyle alone over 12 months in trials — modest compared to GLP-1 medicines but cheaper and oral.

**Side effects** (worse with high-fat meals):

- oily spotting, flatulence, urgent bowel movements

- faecal incontinence if diet is not low-fat

- fat-soluble vitamin deficiency with long-term use — supplements may be advised

**Low-dose orlistat (60mg — Alli):** Available from pharmacies without prescription for BMI 28+. Same mechanism, lower dose.

**Who avoids it:** Malabsorption conditions, cholestasis, pregnancy, breastfeeding.

## Semaglutide — Wegovy

**How it works:** GLP-1 (glucagon-like peptide-1) receptor agonist. Mimics a gut hormone that:

- reduces appetite and increases fullness

- slows stomach emptying

- may reduce food “reward” signalling in the brain

**Form:** **Weekly subcutaneous injection** — pre-filled pen. Dose increased gradually over months to reduce nausea.

**Brand distinction:**

- Ozempic — semaglutide licensed for type 2 diabetes (lower dose range)

- Wegovy — semaglutide licensed for weight management (higher target dose — 2.4mg weekly)

Using Ozempic purely for weight loss in someone without diabetes is **off-label** — NHS weight pathways use **Wegovy** where approved.

**Expected loss:** Trials show average **~15% body weight** loss over 68 weeks vs ~2% with placebo — with lifestyle support.

**Common side effects:**

- nausea (very common initially)

- vomiting, diarrhoea, constipation

- fatigue, headache

- injection site reactions

**Tips:** Eat smaller portions, stop when full, avoid greasy food early on. Nausea usually improves.

**Risks (rare):** pancreatitis, gallbladder disease, bowel obstruction (reported post-marketing), thyroid C-cell tumours in rodent studies (human relevance uncertain — avoid if personal/family history medullary thyroid cancer or MEN2).

## Tirzepatide — Mounjaro

**How it works:** Dual agonist — activates both **GLP-1 and GIP (glucose-dependent insulinotropic polypeptide)** receptors. Strong appetite reduction and metabolic effects.

**Form:** Weekly injection, dose escalated gradually.

**Expected loss:** Trials suggest average **~20% body weight** loss at highest doses over 72 weeks — among the highest seen in obesity pharmacotherapy trials.

**Side effects:** Similar to semaglutide — gastrointestinal symptoms most common, usually improving over time.

**NHS status:** Approved for weight management following NICE guidance — rollout through specialist services similar to Wegovy, subject to supply and local funding.

## How medicines compare

| Medicine | Type | Typical extra loss vs lifestyle | Main side effects |
| --- | --- | --- | --- |
| Orlistat | Oral capsule | Modest (~3–5kg/year) | Oily stools, GI upset |
| Wegovy (semaglutide) | Weekly injection | ~10–15% body weight | Nausea, GI symptoms |
| Mounjaro (tirzepatide) | Weekly injection | ~15–20% body weight | Nausea, GI symptoms |

Individual response varies. Medicines work best with ongoing **behavioural support**, not in isolation.

## What happens when you stop

**Weight regain is common** after stopping GLP-1 medicines — appetite hormones rise again. Long-term strategies include:

- maintained dietary and activity habits

- ongoing support programmes

- in some cases, continued medicine (if criteria still met and benefits outweigh costs)

- bariatric surgery consideration for eligible individuals

Medicines are tools, not permanent fixes without lifestyle change.

## Private prescribing and online supply

Private clinics prescribe Wegovy and Mounjaro outside NHS criteria — costs often **£150 to £300+ monthly**. The **MHRA** has warned about:

- falsified products from unregulated websites

- prescribing without adequate assessment

- sharing pens between users (infection risk, dosing errors)

Use **regulated UK pharmacies and clinics** with proper medical review. Check GPhC registration for online pharmacies.

## Medicines not recommended or unlicensed

**Thyroid hormones (levothyroxine)** for weight loss in people with normal thyroid — dangerous, not prescribed appropriately.

**Ephedrine and DNP (2,4-dinitrophenol)** — DNP has caused deaths; illegal for human consumption.

**“Fat burner” supplements** — poorly regulated, often contain stimulants or banned substances.

**HCG diets** — no evidence, not NHS recommended.

## Monitoring while on medicines

GLP-1 medicines require:

- regular weight and symptom review

- dose titration schedule

- assessment of gallbladder symptoms (rapid weight loss increases gallstone risk)

- blood sugar monitoring if diabetic (doses of other diabetes medicines may need reducing — hypoglycaemia risk)

- mental health check — rare reports of mood changes

Orlistat requires fat-soluble vitamin monitoring with long-term use.

## Weight loss medicines and diabetes

Semaglutide and tirzepatide are also used in **type 2 diabetes** (Ozempic, Mounjaro diabetes brands). If you have diabetes and obesity, one medicine may address both — specialist or GP coordinates dosing and monitors for **hypoglycaemia** if also on insulin or sulfonylureas.

Weight loss itself improves blood sugar — diabetes medication doses often need reducing as weight falls.

## Questions to ask your prescriber

- Do I meet NHS criteria for this medicine?

- What tier service do I need to access first?

- What side effects should I expect and when should I worry?

- How long will I take this medicine?

- What is the plan if I stop — how do we maintain loss?

- How does this interact with my other medicines?

Evidence supports these medicines as **one component** of obesity treatment — alongside nutrition, activity, psychological support, and sometimes surgery. They are not shortcuts around the work of behaviour change, but they can make that work achievable for people whose biology fights against loss.

## ! Important safety information for weight loss medicines

Discuss weight loss medicines with a GP or weight management clinic — not online-only prescribers without full assessment. You need BMI and health assessment, review of eating disorder history, and monitoring plan. Seek urgent help for severe abdominal pain on GLP-1 medicines (rare pancreatitis), allergic reactions, or persistent vomiting preventing fluids.

## Common questions about weight loss medicines

What is the difference between Ozempic, Wegovy and Mounjaro? Ozempic contains semaglutide licensed for type 2 diabetes at lower doses. Wegovy is semaglutide at higher doses licensed specifically for weight management. Mounjaro (tirzepatide) activates both GLP-1 and GIP receptors. On the NHS, weight management uses Wegovy or Mounjaro within approved pathways — not Ozempic prescribed off-label for weight loss alone. How much weight can I lose on Wegovy or Mounjaro? Clinical trials show average losses of roughly 10 to 15% of body weight over 68 to 72 weeks when combined with lifestyle support — individual results vary. Some lose more, some less. Weight often returns if the medicine is stopped without maintained habit changes. What are the side effects of GLP-1 weight loss injections? Very common — nausea, vomiting, diarrhoea, constipation, and reduced appetite (often intended). Usually worst when starting or increasing dose, improving over weeks. Eat smaller meals and avoid fatty foods initially. Rare but serious — pancreatitis, gallbladder problems, allergic reactions. Is orlistat available on the NHS? Yes — orlistat 120mg is prescribed when BMI is 30+ (or 28+ with risk factors) alongside a low-fat diet and exercise plan. A lower dose (60mg) is available from pharmacies without prescription (Alli brand) for adults with BMI 28+. Can I buy Wegovy privately in the UK? Private clinics and some pharmacies prescribe Wegovy and Mounjaro outside NHS criteria — costs are substantial (often hundreds of pounds monthly). Ensure prescribing follows proper assessment — MHRA warns against unregulated online supply. NHS route includes monitoring and criteria assessment. Who should not take weight loss medicines? Pregnant or breastfeeding women, people with history of eating disorders (relative contraindication — needs specialist judgment), certain thyroid cancer history (GLP-1 medicines — medullary thyroid carcinoma, MEN2), severe kidney or liver disease, and those not meeting BMI criteria. Full assessment required. Do weight loss injections affect fertility? Weight loss itself can improve fertility — especially in PCOS. GLP-1 medicines are stopped before planned pregnancy. Orlistat is contraindicated in pregnancy. Discuss family planning with your prescriber. How do I know if I qualify for NHS weight loss medication? Usually BMI 30 or above, or 27.5 or above with weight-related conditions such as type 2 diabetes, high blood pressure, or sleep apnoea — within a specialist weight management programme. Eating disorder history needs careful assessment. GP or clinic calculates BMI and reviews comorbidities. What lifestyle changes work alongside weight loss medicines? NHS programmes combine medicines with dietary advice — calorie deficit, protein intake, reduced ultra-processed food — and increased physical activity. Behavioural support improves long-term results. Medicines reduce appetite but do not replace the need for sustainable habit change.

## Sources

- [NICE — Overweight and obese management](https://www.nice.org.uk/guidance/ng136)
- [NHS — Obesity treatment](https://www.nhs.uk/conditions/obesity/treatment/)
- [MHRA — GLP-1 receptor agonists safety](https://www.gov.uk/drug-safety-update)