Switching from Wegovy to Mounjaro — how to do it safely

Why people switch

Some common reasons we see in consultations:

• Plateau on Wegovy. Weight loss has slowed or stalled at full dose. The hope is that tirzepatide's dual-agonist mechanism pushes things forward again.
• Tolerability issues. Persistent nausea, reflux, or fatigue on semaglutide that hasn't settled with the usual portion-control and slow-eating advice.
• Larger weight loss goal. Head-to-head data favours tirzepatide on average, so switching can be the right step if the goal is meaningfully bigger than what Wegovy has delivered.
• Supply or cost. If the structured programme around Mounjaro suits better, or if supply favours it at the time of the conversation.

None of these are bad reasons. What matters is the switch is planned and supervised — not improvised between online prescriptions.

The planned-gap principle

Both medications are once-weekly. They shouldn't overlap. The standard approach is to take your final Wegovy injection, then leave at least 7 days (one full weekly cycle) before your first Mounjaro injection.

In practice, the gap is often a bit longer — 10 to 14 days — particularly if you've been on a higher Wegovy dose, if you have residual nausea or reflux, or if you're moving from a long stable period at 2.4 mg. A short additional gap is a low-risk way to reduce the chance of stacked GI side effects in the first Mounjaro week.

What you should not do: take your usual Wegovy dose, then take Mounjaro a couple of days later because you're "behind". That stacks two GLP-1-active medications and is the most common mistake patients make when they self-manage a switch.

Starting dose: almost always 2.5 mg

Even if you were on full-dose Wegovy (2.4 mg weekly), the right starting Mounjaro dose for almost everyone is the standard 2.5 mg — the starter dose. Two reasons:

• The receptor profiles aren't equivalent. Semaglutide is GLP-1-only; tirzepatide adds GIP. The gut needs to re-adapt to the combined signal.
• The starter dose minimises the risk of GI side effects during the changeover. Starting higher to "match" your previous Wegovy dose almost never goes well in practice.

Stay on 2.5 mg for the standard 4 weeks, then step up to 5 mg, then continue the normal Mounjaro escalation ladder thereafter (7.5, 10, 12.5, 15 mg as tolerated and indicated).

In a small number of carefully selected patients with strong recent GLP-1 tolerance, a clinician might consider starting at 5 mg. That's a clinical judgement and not the default.

The first 4 weeks after switching

What to expect on 2.5 mg Mounjaro after coming off full-dose Wegovy:

• Appetite signal may briefly dip. 2.5 mg tirzepatide is a starter dose, and after the higher pharmacology of 2.4 mg semaglutide it can feel comparatively quiet for a week or two. This is normal and not a failure of the switch.
• Mild side effects can return. Some patients feel nausea or constipation again — essentially a small reset — then settle within 1–2 weeks.
• Weight is the wrong thing to track week-by-week. The first month after a switch is about getting comfortable on the new medicine. Look at the trend over 3–6 months, not week 2.
• Hydrate and stick to smaller, slower meals. Same playbook as starting tirzepatide from scratch.

What changes structurally

The dose ladder, injection day, side effect profile, and what counts as the appropriate maintenance dose all change. Worth noting:

• Injection day can shift. Many patients use the gap between final Wegovy and first Mounjaro to move to a more convenient day of the week.
• Ladder length. Tirzepatide has more potential steps (2.5, 5, 7.5, 10, 12.5, 15 mg). You don't have to reach 15 mg — the right maintenance dose is the one that delivers the response with tolerable side effects.
• Storage and pen handling. Mounjaro pens and Wegovy pens behave similarly but the instructions aren't identical. Read the new patient information leaflet on day one.

Pitfalls people fall into when switching unsupervised

From clinical experience and from sorting out the consequences:

• Stacking doses. Taking the new medicine while the previous one is still active in the system. Worst-case: severe nausea and dehydration.
• Starting too high. Beginning Mounjaro at 5 mg or above after full-dose Wegovy because "my body's used to it". Usually produces a worse first week.
• Rushing escalation. Trying to get back to a high dose quickly to make up for the perceived drop. Almost always backfires with worse side effects and a higher chance of stopping early.
• No clinical conversation. Switching online without any review of why you're switching, your current tolerability, BMI, comorbidities, and contraindications. This is also the model the MHRA's structured-programme requirements explicitly ruled out.

Switching back, briefly

Mounjaro doesn't suit everyone. The reverse switch (back to Wegovy) follows the same principles: take your final Mounjaro dose, leave at least 7 days (often 10–14 days), restart Wegovy at the standard 0.25 mg starter dose, and re-escalate over 16 weeks. The same warnings apply: don't overlap, don't restart at a high dose.

The role of the structured programme

A switch is the moment a structured programme proves its worth. You should have a consultation before the gap, a check-in around the time of the first new dose, support for any side effects, and a clear plan for escalation. If your current service is just sending pens and isn't able to talk through a switch with you, that's a service-fit issue, not a medication issue.

The next step

If you're thinking about switching from Wegovy to Mounjaro, the most useful single step is a 20–30 minute consultation: it's where the gap gets timed, the starting dose gets agreed, and you find out whether the rest of the programme fits how you live. Bring details of your current dose, when your next injection is due, and any side effects you've had — it makes the conversation faster.