Switching from Wegovy to Mounjaro: A Practical UK Guide
I spent the best part of a year on Wegovy before the scales stopped budging. Not dramatically just a slow, polite plateau where my old mate food noise started tapping me on the shoulder again around 10 PM. If you're reading this, you probably know the feeling.
Here's the short version: yes, switching from Wegovy to Mounjaro is perfectly possible in the UK, and plenty of people do it. But it's not a simple pen-swap on Sunday morning. There's a washout period to observe, a mandatory restart at Mounjaro's lowest dose, and a clinical assessment required before anything leaves the pharmacy. Skip any of those steps and you'll likely spend a weekend wishing you hadn't.
This guide covers why people switch, how the process works step by step, what to expect during the washout week, and, just as importantly, when switching genuinely isn't the right move. No fluff, no miracle-jab noise. Just what I've picked up from helping patients through this exact decision.
Why people switch (and why some shouldn't)
People switch from Wegovy to Mounjaro for three main reasons. The first, and most common, is a weight loss plateau. You've been ticking along nicely on Wegovy, the scales have been cooperating, and then: nothing. Month after month at the same number. Your clinician confirms you're on the maximum 2.4mg dose and there's nowhere left to go within the Wegovy range. That's a fair moment to consider the switch.
The second is side effects. Some people tolerate semaglutide absolutely fine for months, then find the gastrointestinal side effects, nausea, constipation, or that vague queasy feeling that follows you around all day, gradually wear them down. Mounjaro's different mechanism means some people who struggled with Wegovy genuinely get on better with tirzepatide. It's not guaranteed, but it's a valid reason to explore the option.
The third is the clinical data. Mounjaro targets two gut hormones , GLP-1 and GIP, rather than just the one Wegovy works on. The SURMOUNT-1 trial showed patients on the highest Mounjaro dose (15mg) lost up to 20.9% of their body weight over 72 weeks, compared to 14.9% on Wegovy's maximum dose over 68 weeks in the STEP-1 trial. Those numbers don't mean Mounjaro is "better" for everyone, plenty of people do brilliantly on Wegovy long-term, but they do explain why someone who's plateaued might look at the data and wonder.
But here's the bit most guides skip: switching isn't right for everyone. If Wegovy is working well for you, with steady weight loss and tolerable side effects, there is zero clinical reason to switch. Don't let the slightly shinier trial data talk you out of something that's already working. If you've only been on Wegovy for a month or two and haven't given it a fair run at a therapeutic dose, switching is premature. And if you're losing weight steadily on a mid-range Wegovy dose, stick with it , you can always revisit the conversation if things change six months down the line.
The washout period explained
This is the bit that catches people out. You can't take your last Wegovy injection on a Friday and start Mounjaro the following Friday. You need a gap: what clinicians call a washout period.
The rule across virtually every UK provider is a minimum 7-day washout between your last Wegovy dose and your first Mounjaro injection. The reason is straightforward: semaglutide (the active ingredient in Wegovy) has a half-life of roughly one week. That means seven days after your last dose, about 50% of the medication is still circulating in your system. At fourteen days, about 25% remains. It takes four to five weeks for semaglutide to fully clear.
Starting Mounjaro while residual Wegovy is still active creates an overlap effect, two GLP-1 agonists working simultaneously , which can trigger genuinely rough nausea, persistent vomiting, and the kind of dehydration that lands you in A&E. Not worth it.
Some clinicians recommend waiting longer than the 7-day minimum depending on your dose and side effect history. If you were on the maximum 2.4mg Wegovy dose, a two-week washout may be suggested to give your system more breathing room. The key point: follow your prescriber's specific guidance, not a Reddit thread.
During the washout week, most people don't regain weight. Wegovy's appetite-suppressing effects fade gradually rather than disappearing overnight. Days 1 to 3, the medication is still doing its job at roughly 70 to 85% strength. Appetite typically starts creeping back around days 4 to 5. By day 7, you'll likely notice food noise returning, not at full volume, but definitely auditioning for a comeback tour. The habits you built on Wegovy carry you through. Treat the washout week like any other week of your routine, not a holiday from it.
Starting Mounjaro after Wegovy: dosing reality
This is the bit nobody wants to hear: you start Mounjaro at 2.5mg, the same starter dose as someone who's never touched a GLP-1 medication in their life. Even if you've just spent a year on the maximum Wegovy dose. Even if you feel like you've graduated from the starter-pen phase of your life.
Why you can't skip ahead
There's no direct dose conversion between semaglutide and tirzepatide. They're different medications with different mechanisms. Mounjaro targets both GLP-1 and GIP receptors , that second pathway is completely new to your body, regardless of how much Wegovy you've taken. The 2.5mg starter dose exists to let your system adapt to that dual mechanism without throwing a gastrointestinal protest.
I've seen the alternative up close. A patient, let's call him Mark, came to us after trying to speed-run the switch on his own. He'd been on 2.4mg Wegovy, took a four-day "break" (his word), then started himself on 7.5mg Mounjaro he'd sourced elsewhere. He spent the next 48 hours on his bathroom floor, severely dehydrated, and has since described the experience as "the worst hangover of my life multiplied by a stomach bug." He reset to 2.5mg under proper clinical supervision and is now losing weight steadily with zero drama. The monthly titration schedule exists for a reason.
You stay on each Mounjaro dose for a minimum of 4 weeks before stepping up to the next level. The full titration path runs: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg, and potentially 15mg. Your clinician decides the pace based on your response and side effects, not your impatience.
A small number of UK clinics may start patients switching from the highest Wegovy doses at Mounjaro 5mg rather than 2.5mg. This is the exception, not the rule, and only happens after a thorough clinical review. Don't assume you'll be the exception.
What to expect during the switch
Let's be honest about what the first month on Mounjaro actually looks like after Wegovy.
Side effects: a familiar cast, a slightly different show
Mounjaro's side effect profile is similar to Wegovy's, nausea, constipation, diarrhoea, and the occasional episode of vomiting , all concentrated in the early weeks or after dose increases. The difference is that even experienced GLP-1 users sometimes find the first few weeks on Mounjaro a bit more noticeable. Your body already knows semaglutide. Tirzepatide is a stranger, and the GIP component is entirely new territory.
The adjustment period typically lasts 2 to 4 weeks at each dose level. Most side effects settle once your system adapts. Practical tips that genuinely help: eat smaller portions, avoid rich or fried food on injection day, stay hydrated (more than you think you need), and don't lie down immediately after eating. None of these are new ideas, but together they make a meaningful difference.
Weight loss may temporarily slow
The 2.5mg starter dose of Mounjaro is not a therapeutic weight-loss dose, it's an adaptation dose. If you were losing steadily on a high Wegovy dose, the initial Mounjaro titration phase can feel frustratingly slow by comparison. Some people don't notice the full appetite-suppressing effect until they reach 7.5mg or 10mg. This is completely normal and not a sign that switching was the wrong decision. Patience during the first two months is genuinely the hardest part of the switch, and the most important.
Appetite changes feel different
People often describe Mounjaro's effect on food noise as slightly different from Wegovy's, a broader, quieter suppression rather than a sharper mute button. Some prefer it, some don't notice a meaningful difference. Either way, it's worth going in without expecting an identical experience. The dual mechanism means your body is processing the medication differently, and that shows up in how hunger signals feel day to day.
How to switch safely: step by step
Here's the unglamorous, practical version of what the process actually looks like:
1. Book a clinical consultation. You need a fresh assessment before any UK-regulated provider will prescribe Mounjaro to you, even if you're already on Wegovy with the same clinic. The clinician reviews your current dose, treatment history, side effects, and medical suitability for tirzepatide specifically. This takes about ten to fifteen minutes online and isn't a tick-box exercise.
2. Complete the washout week. Wait a minimum of 7 days after your last Wegovy injection. Some clinicians recommend longer, follow their guidance, not your calendar maths. Use the week to keep your eating habits steady and clear a spot in the fridge for your new medication (it needs to stay between 2°C and 8°C until first use).
3. Start Mounjaro at 2.5mg. Inject at the same time of day, on the same day each week, consistency helps your body settle into a rhythm. Rotate your injection site (thigh, abdomen, or upper arm) each week. Don't be the person who injects the same spot every Monday and wonders why it's tender.
4. Stay on each dose for at least 4 weeks. The titration schedule exists to minimise side effects while your body adapts. Your clinician will review your progress before each dose increase. If a dose is working well with manageable side effects, you don't always have to rush to the next one, clinical flexibility matters here.
5. Keep your clinician informed. If side effects are rough, tell them. If you're not feeling any effect at all, tell them. If you're worried about something at 11 PM on a Tuesday, send the message and go to sleep, they'll pick it up in the morning. The patients who have the smoothest transitions are the ones who communicate, not the ones who tough it out in silence.
A note on cost, because nobody likes surprises at checkout. Mounjaro prices at Kensington Slimming Clinic run from £176 per month at the 2.5mg starter dose up to £312 at the maximum 15mg maintenance dose. For comparison, Wegovy runs from £132 to £250 depending on dose. Both prices include the online clinical assessment, the private prescription, the medication, and cold-chain UK delivery in GPhC-registered Woolcool packaging. No hidden consultation fee appearing at checkout. No "admin charge" for the prescription. The number you see is the number you pay.
If you're currently with another provider, you can switch to Kensington Slimming Clinic at any point, you don't need your current provider's permission, and the online assessment takes care of the clinical review. We just need your treatment history so the prescribing clinician knows exactly where you're starting from.
