Semaglutide and tirzepatide are both weekly weight-loss injections, but they work on different numbers of gut-hormone receptors: semaglutide acts on one (GLP-1), while tirzepatide acts on two (GLP-1 and GIP). Neither is automatically "better" for everyone — the right choice depends on your health, tolerance and a prescriber's clinical assessment.
This guide explains what each molecule is, how they differ, what the trials showed, and how they compare on side effects and cost — so you can have a more informed conversation with a UK-registered prescriber.
The two molecules at a glance
Semaglutide is a GLP-1 receptor agonist. It mimics a natural gut hormone that helps control appetite, so you feel fuller sooner and for longer, and your stomach empties more slowly. In the UK it is sold for weight management as the Wegovy injection and also as the once-daily Wegovy Tablet (oral semaglutide).
Tirzepatide is a dual receptor agonist. It activates the GLP-1 receptor and the GIP receptor at the same time. Both are gut hormones involved in appetite and blood-sugar control, and the theory is that engaging two pathways may support appetite regulation differently from GLP-1 alone. In the UK it is sold as Mounjaro.
The key difference in one line
Semaglutide is a single-hormone (GLP-1) medicine; tirzepatide is a dual-hormone (GLP-1 + GIP) medicine. That is the core molecular distinction behind everything else.
How they are taken and titrated
Both injectables are self-administered once a week, and both start at a low "starter" dose that is stepped up gradually. Slow titration gives your body time to adjust and helps limit side effects.
| Medicine | Form | Titration steps | Maximum licensed dose |
|---|---|---|---|
| Semaglutide (Wegovy injection) | Weekly injection | 0.25mg → 0.5 → 1 → 1.7 → 2.4mg | 7.2mg |
| Semaglutide (Wegovy Tablet) | Once-daily tablet | 1.5mg → 4 → 9 → 25mg | 25mg |
| Tirzepatide (Mounjaro) | Weekly injection | 2.5mg → 5 → 7.5 → 10 → 12.5 → 15mg | 15mg |
The Wegovy Tablet is worth a note if injections aren't for you: it is taken first thing on an empty stomach with a small sip of plain water, then you wait at least 30 minutes before eating, drinking anything else, or taking other medicines. Taken with food it is far less effective.
What the clinical trials showed
It is tempting to line up the headline percentages, but these come from separate trials with different participants and designs — they are not head-to-head comparisons. Read them as "what happened in this study", not as a direct scoreboard.
- Tirzepatide: In the SURMOUNT-1 trial, participants on the 15mg dose lost on average 22.5% of body weight over 72 weeks, alongside lifestyle support. Individual results vary; not everyone reaches this.
- Semaglutide (injection): In the STEP UP clinical trial, adults on the 7.2mg dose lost on average 20.7% of their body weight over 72 weeks, alongside lifestyle support. Individual results vary; not everyone reaches this.
- Semaglutide (tablet): In the OASIS 4 trial, adults on oral semaglutide 25mg lost on average 16.6% of their body weight over 64 weeks, alongside lifestyle support. Individual results vary; not everyone reaches this.
Because the trial populations, durations and doses differ, a smaller percentage in one study does not mean a medicine is "weaker" for you personally. What matters more is which treatment you tolerate, can stick with, and which a prescriber judges appropriate for your situation.
Side effects: a shared class profile
Both molecules belong to the same broad GLP-1 family and share a similar side-effect profile. The most common effects (affecting more than 1 in 10 people) include:
- Nausea
- Diarrhoea
- Constipation
- Vomiting
- Decreased appetite
- Fatigue
These are usually most noticeable when you first start or step up a dose, and often settle as your body adjusts — which is exactly why titration is gradual.
Rarer but serious risks for both include severe allergic reaction, acute pancreatitis, severe hypoglycaemia (low blood sugar), severe gastrointestinal disease, and worsening diabetic retinopathy. If you experience side effects, you can report them to the MHRA via the Yellow Card scheme at https://yellowcard.mhra.gov.uk/.
Who they are for
Eligibility is the same across these treatments. They may be an option if you are aged 18 or over and have a BMI of 30 or above, or a BMI of 27–30 with a weight-related condition such as type 2 diabetes, high blood pressure or sleep apnoea.
They are not suitable if you are pregnant, planning pregnancy or breastfeeding; have type 1 diabetes; have a personal or family history of medullary thyroid carcinoma or MEN2; have a history of pancreatitis or severe gastrointestinal disease; have an active eating disorder; have severe liver or kidney impairment; or are allergic to the active ingredient. A prescriber decides whether treatment is clinically appropriate.
How the cost compares
Price depends on the medicine and the dose you reach. Below are the current monthly prices for each; the online consultation is free and you only pay if a treatment is prescribed and you choose to proceed.
| Dose | Wegovy injection | Wegovy Tablet | Mounjaro |
|---|---|---|---|
| Starter | 0.25mg — £115 | 1.5mg — £115 | 2.5mg — £160 |
| Step 2 | 0.5mg — £125 | 4mg — £130 | 5mg — £185 |
| Step 3 | 1mg — £135 | 9mg — £145 | 7.5mg — £240 |
| Step 4 | 1.7mg — £175 | 25mg — £190 | 10mg — £260 |
| Step 5 | 2.4mg — £230 | — | 12.5mg — £275 |
| Top dose | 7.2mg — £245 | — | 15mg — £295 |
Semaglutide options start from £115 a month; Mounjaro starts from £160 a month.
Which should you choose?
There is no universal winner. The decision depends on your medical history, how you respond to and tolerate the medicine, whether you prefer a weekly injection or a daily tablet, and your prescriber's clinical judgement. Some people tolerate one better than the other, and switching is sometimes appropriate under clinical supervision.
The best next step is a proper assessment. You can compare the full range on our treatments overview and weight-loss injections page, or begin a free eligibility check at start your consultation. A GPhC-registered prescriber will review your answers and recommend a treatment only if it is clinically appropriate — a prescription is never guaranteed.
These are Prescription-Only Medicines and are available only after a clinical consultation with a GPhC-registered prescriber. To read more about how our regulated UK service works, see our about page or browse the blog.
This article is for information only and is not a substitute for personalised medical advice. Weight-loss medicines are prescription-only, are not suitable for everyone, and are prescribed only after a clinical consultation with a GPhC-registered prescriber. Individual results vary. Always speak to a UK-registered pharmacist or doctor before starting or changing any prescription medication.
