Semaglutide vs Tirzepatide: How Do They Compare?

Semaglutide and tirzepatide are both injectable prescription medicines approved in Australia for type 2 diabetes and, in certain formulations, weight management. The key difference is their mechanism: semaglutide acts on a single gut-hormone receptor (GLP-1), while tirzepatide acts on two (GIP and GLP-1). This article is general educational information, not medical advice.

What are semaglutide and tirzepatide?

Both are “incretin” medicines, they mimic gut hormones that help regulate blood sugar and appetite. In Australia, both are TGA-approved, prescription-only medicines, not research chemicals.

• Semaglutide is marketed as Ozempic (type 2 diabetes) and Wegovy (weight management).
• Tirzepatide is marketed as Mounjaro.

Because they are registered medicines, they go through the same approval, manufacturing and pharmacovigilance standards as other prescription drugs, which is a meaningful difference from the unapproved research peptides discussed elsewhere on this site, such as BPC-157.

How do they work?

The two medicines differ in how many incretin receptors they target. Semaglutide is a single-pathway drug; tirzepatide is a dual-pathway drug.

Semaglutide

Semaglutide is a GLP-1 receptor agonist. By activating the GLP-1 receptor, it is reported to slow stomach emptying, increase feelings of fullness, and improve the body’s insulin response to food.

Tirzepatide

Tirzepatide is a dual GIP and GLP-1 receptor agonist. It activates the GLP-1 receptor and the GIP receptor. This dual action is the structural reason it is often described as a different class, and is thought to underlie the larger average effects reported in some trials.

Semaglutide vs tirzepatide: full comparison

The table below summarises the main differences. Reported effects vary between individuals; figures are general and not a prediction for any one person.

Feature	Semaglutide	Tirzepatide
Drug class	GLP-1 receptor agonist	Dual GIP + GLP-1 receptor agonist
AU brand names	Ozempic, Wegovy	Mounjaro
AU approval status	TGA-approved, prescription-only	TGA-approved, prescription-only
Approved uses	Type 2 diabetes; weight management (Wegovy)	Type 2 diabetes; weight management
Typical dosing frequency	Once weekly (injectable)	Once weekly (injectable)
Reported effects	Reduced appetite, improved glucose control, weight reduction	Reduced appetite, improved glucose control, often larger reported weight reduction
Reported side effects	Nausea, vomiting, diarrhoea, constipation	Nausea, vomiting, diarrhoea, constipation
Pathways targeted	One (GLP-1)	Two (GIP + GLP-1)

A note on “which is better”

Head-to-head trials have reported greater average weight reduction with tirzepatide compared with semaglutide. But “better” depends entirely on the indication, the individual, tolerability and clinical context, which is a conversation for a qualified prescriber, not a blog.

What side effects are reported?

Both medicines share a similar, mostly gastrointestinal side-effect profile. The most commonly reported issues are nausea, vomiting, diarrhoea and constipation, which are often described as more pronounced when starting or increasing a dose.

More serious but less common concerns described in product information for this class include:

• Pancreatitis (inflammation of the pancreas)
• Gallbladder problems
• Low blood sugar (particularly when combined with certain other diabetes medicines)
• Thyroid C-cell tumour warnings (based on rodent studies; relevance to humans is part of the labelled precautions)

These are summarised for education only. Anyone prescribed these medicines should rely on their doctor, pharmacist and the official product information, not a comparison article, for guidance on side effects and contraindications.

Why does the compounded / grey-market version matter?

Because demand has outstripped supply at times, compounded and grey-market versions of these molecules have appeared. These sit outside the standard registered-medicine pathway and carry real, documented risks.

The core problems are:

• Unknown dosing - the actual amount of active ingredient may differ from the label.
• Wrong or extra ingredients - products may contain the wrong salt form, the wrong peptide, or impurities.
• No manufacturing oversight - registered medicines are made under strict quality standards; grey-market product often is not.

This is exactly where testing matters. If a product is not a registered, pharmacist-dispensed medicine, the only way to know what it actually contains is independent analysis. Our guide on how to read a peptide test result explains how purity (HPLC) and identity (mass spectrometry) are verified, and why a label alone tells you nothing reliable.

Are they legal in Australia?

The registered medicines (Ozempic, Wegovy, Mounjaro) are legal when prescribed by a doctor and dispensed by a pharmacist. Unapproved, compounded or imported versions are a different legal matter entirely.

For the regulatory detail, prescription-only scheduling, the personal importation scheme, and the difference between possessing, importing and supplying, see Are peptides legal in Australia?

The bottom line

Semaglutide and tirzepatide are both TGA-approved, once-weekly injectable prescription medicines with overlapping uses and a similar side-effect profile, differing mainly in mechanism: semaglutide targets GLP-1, while tirzepatide targets both GIP and GLP-1. Reported differences in effect come from clinical trials and vary by individual.

This is general, educational information and not medical advice. Decisions about these medicines belong with a qualified prescriber. And if a product is anything other than a registered, pharmacist-dispensed medicine, treat the label with scepticism and consider independent laboratory testing before drawing any conclusions about what it contains.