Semaglutide vs Tirzepatide: Key Differences for Prescribers

With both semaglutide and tirzepatide now available for weight management, prescribers face a practical question: which drug, for which patient? This article breaks down the key differences across pharmacology, trial data, side effects, and real-world prescribing considerations.

Pharmacological mechanism

The fundamental difference is in receptor activity.

Semaglutide is a selective GLP-1 receptor agonist. It mimics the native incretin hormone GLP-1, producing appetite suppression, delayed gastric emptying, and glucose-dependent insulin secretion.

Tirzepatide is a dual GIP/GLP-1 receptor agonist — sometimes called a “twincretin.” In addition to GLP-1 receptor activation, it also activates the GIP (glucose-dependent insulinotropic polypeptide) receptor. GIP agonism appears to enhance insulin sensitivity and may contribute additional effects on energy balance and fat metabolism, though the precise mechanisms are still under investigation.

In short, tirzepatide has a broader receptor pharmacology. Whether this translates to meaningfully better outcomes for every patient is a more nuanced question.

Efficacy: what the trials show

Semaglutide — the STEP programme

The STEP trials evaluated semaglutide 2.4 mg weekly in adults with obesity or overweight with comorbidities. Key results:

STEP 1 — 16.9% mean body weight loss at 68 weeks versus 2.4% with placebo
STEP 3 (with intensive behavioural therapy) — 16.0% mean weight loss
STEP 5 (two-year data) — 15.2% sustained weight loss at 104 weeks

Roughly one-third of participants achieved 20% or greater weight loss on semaglutide.

Tirzepatide — the SURMOUNT programme

The SURMOUNT trials evaluated tirzepatide at 5 mg, 10 mg, and 15 mg weekly doses:

SURMOUNT-1 — mean weight loss of 15.0% (5 mg), 19.5% (10 mg), and 20.9% (15 mg) at 72 weeks versus 3.1% with placebo
SURMOUNT-2 (in T2DM patients) — 12.8% to 14.7% weight loss depending on dose
Over half of participants on the 15 mg dose achieved 20% or greater weight loss

Head-to-head

The SURMOUNT-5 trial directly compared tirzepatide 15 mg to semaglutide 2.4 mg. At 72 weeks, tirzepatide produced a mean weight loss of 20.2% versus 13.7% for semaglutide — a statistically significant difference of approximately 47% greater relative weight reduction.

On pure efficacy numbers, tirzepatide at the highest dose outperforms semaglutide. However, efficacy is only one factor in prescribing decisions.

Titration schedules

Both drugs use gradual dose escalation to manage gastrointestinal tolerability.

Semaglutide (Wegovy): 0.25 mg → 0.5 mg → 1.0 mg → 1.7 mg → 2.4 mg, escalating monthly. Five steps over approximately 16–20 weeks to reach maintenance.

Tirzepatide (Mounjaro): 2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg, escalating monthly. Six steps, with more flexibility — 5 mg, 10 mg, and 15 mg are all recognised as potential maintenance doses depending on tolerability and response.

Tirzepatide’s broader range of maintenance doses gives prescribers more granularity. A patient who responds well at 10 mg with good tolerability may not need to escalate to 15 mg, whereas with semaglutide, the target is typically 2.4 mg for weight management.

Side effect profiles

Both drugs share a common side effect profile driven by their GLP-1 activity:

Nausea — the most frequently reported adverse event for both drugs, typically worst in the first few weeks at each dose level
Vomiting and diarrhoea — common but usually transient
Constipation — reported by approximately 10–15% of patients on either drug
Injection site reactions — generally mild

In the SURMOUNT and STEP trials, discontinuation rates due to adverse events were broadly similar: around 4–7% for both drugs. Tirzepatide may produce slightly higher rates of diarrhoea, while semaglutide tends to produce marginally more nausea at equivalent efficacy doses, though differences are modest.

The clinical reality is that side effects for both drugs are most intense in the days following injection — correlating with peak plasma concentrations — and during dose escalation phases. This is where structured daily check-ins prove valuable: catching a patient’s nausea on day two of a new dose is far more useful than hearing about it at a monthly review.

Cost and availability

Pricing varies by supplier and setting, but in general tirzepatide carries a higher list price than semaglutide. For many private prescribers and pharmacies, patient cost is a significant factor in treatment selection. A patient who cannot sustain the cost of tirzepatide long-term may achieve better outcomes on semaglutide simply through better adherence.

Supply reliability is also worth considering. Both drugs have experienced intermittent supply constraints in the UK market since their respective launches.

Choosing between them

There is no universal answer. Practical considerations include:

Efficacy expectations — for patients with higher BMI or greater weight loss targets, tirzepatide’s superior efficacy data may justify the higher cost
Tolerability history — a patient who has struggled with GI side effects on semaglutide may respond differently to tirzepatide, or vice versa
Cost sensitivity — for long-term treatment, the drug the patient can afford and will continue taking is the right choice
Dose flexibility — tirzepatide offers more maintenance dose options for fine-tuning

Whichever drug you prescribe, the support infrastructure around it matters just as much. Cadence helps prescribers across pharmacies, clinics, and online services monitor patients through each titration step, track side effects against the dosing cycle, and make data-informed decisions about escalation. If you would like to see how this works for your setting, book a demo.