Your Wegovy pen is nearly empty. You've already paid for Mounjaro and it's sitting in your fridge. The temptation to just crack on with the new medication is strong. Why wait around when you could start seeing better results immediately?

We get it. Every day without medication feels like a day your appetite could spiral back out of control. But switching straight away isn't just inadvisable. It could make you feel genuinely awful and set your treatment back weeks.

The direct answer: No, you shouldn't switch from Wegovy to Mounjaro straight away. You need a minimum 7-day gap between your last Wegovy injection and your first Mounjaro dose. Here's exactly why that matters and how to make the transition as smooth as possible.

Both Wegovy and Mounjaro work on your GLP-1 receptors. Think of these receptors like parking spaces. Wegovy (semaglutide) is already occupying those spaces when you take it. If you add Mounjaro (tirzepatide) immediately, you're essentially trying to park two cars in the same spot.

The result? Overstimulation of those receptors, leading to:

• Severe nausea that goes far beyond the mild queasiness you might have experienced when starting Wegovy
• Persistent vomiting that could leave you dehydrated and unable to keep food down
• Intense stomach cramps and digestive discomfort
• Diarrhoea or constipation worse than anything you've experienced on treatment
• Complete loss of appetite to the point where even water feels nauseating

These aren't rare complications. They're the predictable outcome of doubling up on GLP-1 stimulation. Severe GI side effects from GLP-1 medications can occasionally require medical attention if dehydration develops, and overlapping two medications significantly increases that risk.

Semaglutide (Wegovy's active ingredient) has a half-life of approximately 7 days. In practical terms, this means:

• 7 days after your last injection: 50% of the medication remains in your system
• 14 days after: 25% remains
• 21 days after: 12.5% remains
• 4-5 weeks after: Medication is essentially cleared

The 7-day minimum gives your body enough time for Wegovy levels to drop significantly, even though some medication remains. By day 7, your GLP-1 receptors have enough capacity to handle tirzepatide without dangerous overstimulation.

Some patients convince themselves they'll be fine. They've tolerated Wegovy well, they reason, so surely their body can handle a bit of overlap.

The problem is that tolerating one GLP-1 medication doesn't mean your body can handle two at once. Mounjaro isn't just another GLP-1 agonist. It's a dual-action medication that also targets GIP receptors. Adding it on top of lingering Wegovy creates a triple effect your digestive system isn't prepared for.

Patients who've switched too quickly report:

• Days of being unable to eat anything
• Missing work due to constant nausea
• Needing anti-emetics (anti-sickness medication) for the first time
• Abandoning Mounjaro entirely because they assume the medication doesn't suit them
• Needing medical attention for dehydration if vomiting becomes severe

That last point is particularly frustrating. Patients give up on Mounjaro thinking it's the wrong medication for them, when actually it was the overlap that caused the problems. With a proper washout period, they might have tolerated it perfectly well.

If you're determined to switch, here's how to do it properly:

The fear of the washout period is usually worse than the reality. Here's what actually happens and how to handle it:

Days 1-3: Business as usual. Wegovy is still at 70-85% strength. Most patients notice zero difference in appetite or cravings.

Days 4-5: You might start thinking about food more often. This isn't failure. It's just your medication levels dropping as expected. Stick to your meal routine.

Days 6-7: Appetite may feel closer to pre-treatment levels, but you're nearly there. Focus on protein-rich meals that keep you satisfied longer.

This is where having an experienced provider matters enormously. Someone new to GLP-1 medications always starts Mounjaro at 2.5mg. But you're not new. Your body has already adapted to these medications.

Depending on your Wegovy dose and how well you tolerated it, your clinician may recommend starting higher:

• Previously on Wegovy 0.25-0.5mg: Likely start at Mounjaro 2.5mg
• Previously on Wegovy 1mg: May start at Mounjaro 2.5mg or 5mg
• Previously on Wegovy 1.7-2.4mg: May start at Mounjaro 5mg or even 7.5mg

Starting at an appropriate dose means you won't spend weeks on a dose that barely suppresses your appetite. It also reduces the frustration of feeling like you've gone backwards in your treatment.

If the switch requires a week without medication and potential side effects as you adjust, why bother?

For many patients, the answer is results. Clinical trials show Mounjaro delivers superior weight loss compared to Wegovy:

• Mounjaro (SURMOUNT-1 trial): Up to 22.5% body weight loss over 72 weeks
• Wegovy (STEP-1 trial): Approximately 15% body weight loss over 68 weeks

That 5-7% difference might not sound dramatic, but for someone weighing 15 stone, it represents an additional stone of weight loss. For many patients, especially those who've plateaued on Wegovy, Mounjaro offers a path to reaching their goal weight.

Other reasons patients switch:

• Weight loss has stalled despite being on maximum Wegovy dose
• Persistent side effects on Wegovy that might improve with a different medication
• Wegovy stock shortages making consistent treatment difficult
• Additional blood sugar benefits from Mounjaro's dual action

Making the switch safely means working with a provider who understands the transition process and won't just hand you a starting dose prescription without considering your history.

G-1 Nutrition provides:

• Individualised dosing based on your Wegovy history and tolerance
• Clinical oversight from GPhC-registered pharmacists who understand these medications
• Clear guidance on managing the washout period
• Responsive support if you experience problems after starting Mounjaro
• Transparent pricing with no hidden fees

If you would like to learn more, we've created a comprehensive guide here that walks you through the step-by-step process on how to swap your Mounjaro or Wegovy provider.