Why patients switch from Wegovy to Mounjaro

Most people who switch have been on Wegovy for six months or longer. They've lost some weight — often 8 to 15 percent of body weight — but they've either plateaued, or never quite got the loss they hoped for. Mounjaro (tirzepatide) became available privately in the UK in 2024, and the SURMOUNT trials suggested a higher average weight loss than the STEP trials for Wegovy.

Three common reasons we see patients switch:

• Plateau — you've been stuck at the same weight for eight to twelve weeks on the maximum Wegovy dose (2.4mg)
• Side-effects you can't tolerate — Wegovy gave you persistent nausea, diarrhoea or fatigue that didn't resolve after the first month
• You want the higher average loss — SURMOUNT-1 reported a mean body weight loss of 22.5 percent on tirzepatide 15mg over 72 weeks, versus 14.9 percent on semaglutide 2.4mg in STEP-1 (different trials, but the comparison is reasonable)

Is switching safe?

Yes. Both medications act on the GLP-1 receptor pathway (Mounjaro also activates the GIP receptor), and there's no clinical conflict in moving from one to the other. The transition is well-tolerated in clinical practice. The two principles that matter:

• Don't overlap the two medications
• Start Mounjaro at the lowest dose (2.5mg) even if you were on the maximum Wegovy dose

How the switch actually works

The protocol we follow at Preston Clinic:

1. Last Wegovy dose — your usual weekly dose at the usual time
2. Wait seven days — this is your normal dosing gap. No special washout period needed
3. First Mounjaro dose — 2.5mg, on what would have been your next Wegovy day
4. Escalate — standard Mounjaro titration: 2.5mg for four weeks, then 5mg, 7.5mg, 10mg, 12.5mg, 15mg as tolerated. Most patients reach 10mg or 15mg as their maintenance dose

You may experience some mild gastrointestinal side-effects in the first one to two weeks of Mounjaro 2.5mg, even though you tolerated Wegovy 2.4mg without issue. That's normal — your body is adapting to a slightly different molecule.

Will I lose more weight on Mounjaro?

On average, yes — but the honest answer is 'it depends.' The SURMOUNT-1 trial showed 22.5 percent mean body weight loss at 72 weeks on Mounjaro 15mg, versus 14.9 percent in STEP-1 for Wegovy 2.4mg. But:

• These were different trials with different patient populations, so direct comparison is imperfect
• Individual response varies — some people respond better to semaglutide than tirzepatide
• If you've already lost significant weight on Wegovy, your further loss on Mounjaro will be smaller than someone starting from baseline

What to bring to your switching consultation

• Your current Wegovy pen and any remaining stock
• A note of when you took your last dose
• Weight loss to date and any side-effects you've experienced
• Any other medications you're on, including over-the-counter supplements

Hamza will review your eligibility against NICE TA1026 (tirzepatide for obesity), confirm the switch makes clinical sense, and issue your first Mounjaro dose the same visit if appropriate. The whole appointment usually takes 30 minutes.

When NOT to switch

Switching isn't always the right call. If Wegovy is working — you're still losing weight, side-effects are manageable, you're not at plateau — there's no advantage to changing. Mounjaro is not 'better,' it's different. Most patients who plateau on Wegovy do see further loss on Mounjaro, but not everyone, and a poorly-tolerated switch can be more disruptive than staying on a treatment that's working for you.