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Why people switch from Wegovy to Mounjaro
The most common reason is a plateau. Wegovy did its job for a while, then the scales went quiet and the appetite started creeping back in around 9pm. When that happens, people start asking whether a medicine that targets two hormones might do what one hormone stopped doing.
That is the clinical difference in a sentence. Wegovy is semaglutide, and it mimics one gut hormone, GLP-1. Mounjaro is tirzepatide, and it mimics two, GLP-1 and GIP. Two levers instead of one.
The trial numbers back it up. In the SURMOUNT-1 trial, people on Mounjaro 15 mg lost up to 20.9% of their body weight over 72 weeks. In the STEP-1 trial, Wegovy 2.4 mg delivered up to 14.9% over 68 weeks. Both are licensed weight-management medicines, with guidance from NICE. Mounjaro just averages higher.
Some people switch for a different reason. Wegovy suits them fine, but the nausea will not settle, or they cannot get their dose in stock. Others have simply heard about the extra hormone and want a look. All valid, but the reason shapes how you switch.
You don't convert the dose, you restart it
Here is the part most people get wrong. They assume switching from Wegovy to Mounjaro works like swapping a 2.4 mg dose for an equivalent. It does not. There is no conversion table between semaglutide and tirzepatide. They are different molecules.
So you do not pick up where you left off. You go back to the start. Mounjaro begins at 2.5 mg once a week, whatever dose of Wegovy you were on. From there it steps up by 2.5 mg every four weeks, as long as your clinician is happy, until you reach the dose that works.
I have watched people try to skip that ladder. One patient switched from a high Wegovy dose and decided to jump straight up the Mounjaro scale, convinced it would speed things along. It did not. It spent a weekend on the bathroom floor. We reset him to the standard monthly steps, and he lost weight steadily with no drama.
The ladder is there for a reason. Your body needs time to meet the new GIP mechanism. Skip a rung and the reward is nausea, not faster results. Rule of thumb: start low, go slow, and let the medicine do the walking.
The one-week gap, and never taking both
Between your last Wegovy dose and your first Mounjaro dose, most prescribers want a gap of around seven days. Semaglutide has a half-life of about a week, so this gives your system time to clear most of the old medicine before the new one arrives.
The reason is simple. If the two overlap, you are effectively taking a double dose of two appetite-suppressing medicines at once. That is how you end up with severe vomiting and dehydration rather than steady weight loss.
So never take Wegovy and Mounjaro together, and never restart Mounjaro the same week you stopped Wegovy without a clinician telling you to. I cannot say this too plainly. If you are mid-switch and you are not sure, speak to the person who prescribed it.
For the warning signs to watch during any dose change, our guide to slimming injections side effects covers what is normal and what needs a doctor.
Side effects to expect when you switch
Even if Wegovy treated you gently, expect a few digestive grumbles in the first couple of weeks on Mounjaro. Nausea, constipation, and loose stools are the usual trio. The GIP mechanism is new to your body, so it takes a moment to settle in.
You may also notice a temporary stall. Some people gain a pound or two in the first month as the old medicine clears and the new one is still at its lowest dose. That is normal. It is not the treatment failing. It is the transition.
Your appetite might nudge back in briefly, too. That is the semaglutide leaving and the tirzepatide still ramping up. It settles as the dose climbs. Most people need a few months at the working dose before they can judge whether Mounjaro beats Wegovy for them.
When switching isn't the right move
Here is where I talk some of you out of it. If Wegovy is working and you are just impatient, stay put. You do not always have to race to the highest dose, and you do not have to chase the biggest trial number. A medicine that is working quietly is worth more than a new one you are guessing at.
And if it is about money, know the maths before you jump. Mounjaro costs more. It starts at £174.99 a month and climbs to £309.99 at the top dose. Wegovy runs from £129.99 to £249.99. That extra hormone comes at a price.
Mounjaro is also not right for everyone. If you have had pancreatitis or certain thyroid conditions, it is off the table. If you are pregnant or breastfeeding, the same. Only a proper clinical assessment catches that. You can check current eligibility on the NHS website, but a switch should always go through a clinician, never a DIY plan.
If you want to compare the two side by side before deciding, start with our breakdown of the best weight loss injections UK. And if you decide Mounjaro is the one, read how the Mounjaro treatment works in more detail.
