Semaglutide (single GLP-1 agonist) and Tirzepatide (dual GIP/GLP-1) are the two dominant incretin peptides in Australia. They look similar on a vial label but are different molecules with different masses, different dose ladders, and different purity benchmarks.
Semaglutide
Tirzepatide
Receptor target
GLP-1 only
GIP + GLP-1 dual agonist
Molecular weight
~4114 Da
~4814 Da
Half-life
~7 days
~5 days
Typical research dose
0.25 → 2.4 mg weekly
2.5 → 15 mg weekly
Mean weight loss (72 wk trials)
~15% (STEP)
~21% (SURMOUNT-1)
Identity (LC-MS) expected mass
4113.6 Da
4813.5 Da
HPLC purity benchmark
≥ 98%
≥ 98%
Common AU mislabel risk
Under-fill vs label claim
Sold as semaglutide at higher price
Quantitative content check
Required (mg/vial)
Required (mg/vial)
Verdict
Tirzepatide produces larger weight-loss outcomes in head-to-head trials. Semaglutide has a longer real-world track record and lower per-mg cost. For QC, the single biggest risk on the Australian grey market is dose under-fill - a CoA with quantitative HPLC mg/vial is non-negotiable for either.
Frequently asked questions
Can a vendor swap one for the other?
It happens - they look identical as lyophilised powder. LC-MS identity catches it instantly because the masses differ by ~700 Da.
Which has more counterfeit risk in Australia?
Tirzepatide, because per-mg price is higher and demand outpaces global supply. Always test a first-order vial from any new vendor.
Does LabDoc test both?
Yes - same A$549 protocol, same identity + purity + quantitative content report.