Ozempic, Wegovy, Mounjaro, Zepbound: which is which

The single most common source of confusion in this category is that the same molecule is sold under two names, licensed for two different things, and discussed as though it were two different drugs. It is worth ten minutes to get straight, because it changes what you should be writing down.

Why one molecule gets two names

A medicine is approved for an indication, not in general. When a manufacturer runs a separate trial programme for a second indication, the result is usually a second brand, its own packaging, its own dose range and its own pens. Ozempic and Wegovy are both semaglutide; Wegovy exists because semaglutide was studied and approved for weight management at doses that go higher than the diabetes range. Mounjaro and Zepbound stand in exactly the same relationship for tirzepatide.

For a record, the consequence is simple and worth adopting straight away: write down the molecule and the milligrams, not the brand. Brands get switched by supply, by insurance, by country. The molecule does not, and the milligrams are what carry over.

Where the two molecules genuinely differ

Semaglutide is a GLP-1 receptor agonist. Tirzepatide acts at two receptors, GLP-1 and GIP, which is why it is usually described as a dual agonist rather than "a stronger Ozempic". They are not interchangeable at equivalent milligram numbers, and the dose ladders are different: semaglutide steps through fractions of a milligram, tirzepatide through 2.5 mg increments. Copying a number across from one to the other is meaningless.

They also differ in how long they persist. Semaglutide has a terminal half-life of roughly seven days; tirzepatide around five. Both are weekly injections, and both take about a month at a new dose before the amount circulating settles, which is the reason dose steps are spaced four weeks apart rather than fortnightly. The mechanics of that are covered in peptide half-life, explained.

What changes when you switch brands

Switching between two brands of the same molecule at the same milligram dose is a supply event rather than a clinical one, but it is still worth a line in the log: pens differ in how they are dialled and in how many doses they hold, and a switch is a common moment for a dose to be misread. Switching molecules, semaglutide to tirzepatide say, is a different matter entirely: it usually means restarting a titration from the bottom of the new ladder, and that decision belongs to whoever prescribes it.

Either way, the record should show the change explicitly, with the date. Six months later, a weight curve with an unexplained inflection in it is a puzzle. The same curve with "switched to tirzepatide 2.5 mg" marked on the day is an answer.

A note on what is in the box

This class has a large grey market attached to it: compounded versions, unlicensed vials, and material sold as "research use only" that is clearly not being bought for research. It is not this page's job to lecture, but it is worth saying the practical thing plainly. If what is in the vial is not verified, then the dose you record is a hypothesis, the level any app estimates from it is a hypothesis built on a hypothesis, and side effects cannot be attributed to anything with confidence. A record is only ever as good as the certainty about what went in.