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UK Prescription Weight Loss Medications: Comparing Orlistat, Wegovy, Mounjaro, and Saxenda
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- Metabolic Boost Diets Editorial Team
Weight loss medications are regulated as medicines in the UK — not supplements. Each requires a prescription, has been through MHRA (Medicines and Healthcare products Regulatory Agency) approval, and has a NICE-evaluated evidence base. Comparing them requires understanding what they do, how much weight loss they produce, what side effects they carry, and who can access them.
The Four Licensed UK Weight Loss Medications
Orlistat (Xenical 120mg / Alli 60mg)
Mechanism: Lipase inhibitor. Blocks approximately 25–30% of dietary fat absorption in the intestine. The unabsorbed fat passes through to the colon.
Evidence: Davidson et al. (1999, JAMA, RCT, n=892, 2 years): orlistat 120mg three times daily plus diet produced approximately 10.3% weight loss vs. 6.1% with diet alone — approximately 4.2% additional weight loss vs. placebo.
Side effects: Oily/greasy stools, oily spotting, flatulence with discharge, urgent or frequent bowel movements. These are not rare — they occur in most users and are caused by the mechanism of action. Severity is directly related to fat intake: keeping meals below 15g fat significantly reduces GI effects.
Vitamin absorption: Orlistat reduces absorption of fat-soluble vitamins (A, D, E, K). A multivitamin taken at least 2 hours before or after orlistat is recommended throughout treatment.
NICE eligibility: BMI ≥30 (or ≥28 with comorbidities). Typically initiated in Tier 2 weight management services or by GP. Alli 60mg is available OTC in pharmacies for BMI ≥28.
NHS availability: Yes — widely available through GPs and Tier 2 services.
Cost (private): Alli OTC approximately £30–50/month; Xenical 120mg approximately £50–80/month.
Liraglutide 3mg (Saxenda)
Mechanism: GLP-1 receptor agonist. Binds to GLP-1 receptors in the pancreas, gut, and brain: reduces appetite, slows gastric emptying, increases satiety signalling.
Evidence: Davies et al. (2015, SCALE trial, Lancet, RCT, n=3,731, 56 weeks): liraglutide 3mg produced mean weight loss of 8.4% body weight vs. 2.8% with placebo — approximately 5.6% additional loss.
Dose escalation: 0.6mg/day for week 1, increasing by 0.6mg/week to target dose of 3mg/day by week 5. This reduces nausea on initiation.
Side effects: Nausea (most common, particularly during dose escalation), vomiting, diarrhoea, constipation. These typically improve over 4–8 weeks. Rare: pancreatitis (reported, causal relationship uncertain), gallstones (weight loss itself increases gallstone risk).
Contraindications: Personal or family history of medullary thyroid carcinoma or MEN-2 (multiple endocrine neoplasia type 2).
NICE eligibility: NICE TA664 (2020): liraglutide recommended for BMI ≥35 (or ≥32.5 in certain ethnic groups) with a weight-related comorbidity, after failure of at least one Tier 2 programme.
NHS availability: Via Tier 3 specialist weight management services.
Cost (private): Approximately £200–250/month.
Semaglutide 2.4mg weekly (Wegovy)
Mechanism: GLP-1 receptor agonist (same class as Saxenda but semaglutide has higher binding affinity and weekly dosing).
Evidence: Wilding et al. (2021, STEP 1 trial, NEJM, RCT, n=1,961, 68 weeks): semaglutide 2.4mg weekly produced mean weight loss of 14.9% body weight vs. 2.4% with placebo — approximately 12.5% additional loss.
At the top end: 32% of participants lost ≥20% of body weight.
Dose escalation: 0.25mg/week for 4 weeks, then increases over 16 weeks to 2.4mg/week maintenance dose.
Side effects: Nausea, vomiting, diarrhoea, constipation (same GLP-1 class effects as Saxenda, often milder due to slower dose escalation). Nausea is most common during dose escalation phase.
Advantages over Saxenda: Weekly injection vs. daily; approximately double the weight loss (14.9% vs. 8.4%); improved adherence with weekly dosing.
NICE eligibility: NICE TA875 (2023): semaglutide recommended for BMI ≥35 (or ≥32.5 for certain ethnic groups) with at least one weight-related comorbidity, within a specialist weight management service. Implementation phased due to supply constraints.
NHS availability: Via specialist weight management services; access varies by ICB region. Private prescribers (online clinics) have driven significant private uptake.
Cost (private): Approximately £200–260/month.
Tirzepatide 15mg weekly (Mounjaro)
Mechanism: Dual GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 receptor agonist. The additional GIP receptor activation is believed to enhance weight loss beyond GLP-1 alone — possibly through increased energy expenditure and fat oxidation.
Evidence: Jastreboff et al. (2022, SURMOUNT-1 trial, NEJM, RCT, n=2,539, 72 weeks): tirzepatide 15mg produced mean weight loss of 20.9% body weight vs. 3.1% with placebo. 57% of participants lost ≥20% of body weight.
Dose escalation: 2.5mg/week for 4 weeks, increasing every 4 weeks to maximum 15mg/week.
Side effects: Similar to semaglutide (nausea, GI effects during escalation). The dual mechanism does not appear to increase side effect burden significantly.
NICE eligibility: NICE TA1026 (2024): tirzepatide approved for similar criteria to semaglutide (BMI ≥35 with comorbidities within specialist services). Also has Type 2 diabetes indication (Mounjaro) at lower doses.
NHS availability: Specialist weight management services; also prescribed privately at scale.
Cost (private): Approximately £200–280/month.
Comparison Table
| Medication | Mechanism | Mean weight loss | Administration | NHS eligible? |
|---|---|---|---|---|
| Orlistat 120mg | Fat absorption inhibitor | +4.2% vs. placebo (~10% total) | Oral, 3×/day with meals | Yes (Tier 2) |
| Liraglutide 3mg (Saxenda) | GLP-1 agonist | ~8.4% total | Daily injection | Tier 3 |
| Semaglutide 2.4mg (Wegovy) | GLP-1 agonist | ~14.9% total | Weekly injection | Tier 3 (phased) |
| Tirzepatide 15mg (Mounjaro) | Dual GIP/GLP-1 agonist | ~20.9% total | Weekly injection | Tier 3 |
How to Access These Medications in the UK
NHS pathway:
- GP assessment (BMI, comorbidities, previous attempts)
- Tier 2 referral for Tier 2 programme (commercial weight management, group sessions)
- If Tier 2 fails: Tier 3 referral for specialist weight management service (where GLP-1 medications are prescribed)
- Tier 4: bariatric surgery for BMI ≥40 or ≥35 with comorbidities
Waiting times for Tier 3 services vary significantly by region.
Private prescribers: Multiple UK online clinics (Numan, Voy, Juniper, Manual) provide private prescriptions for semaglutide and tirzepatide with structured clinical support. This requires:
- BMI ≥30 (typically)
- GP-level clinical assessment
- Ongoing prescription review
Never purchase injectable weight loss medications from non-prescriber sources. Counterfeit semaglutide and tirzepatide products have been identified in the UK market (MHRA warning 2024). These can be contaminated, mislabelled, or contain incorrect doses.
Disclaimer: This article is for informational and educational purposes only. Weight loss medications require medical assessment and prescription. Consult your GP or a licensed UK prescriber. Do not use injectable weight loss medications without medical supervision.