If you’re considering weight loss injections, chances are you’ve come across Mounjaro and Wegovy—two of the most talked-about treatments available today. Both are weekly injections designed to help people with a high BMI lose weight safely and sustainably. But one of the biggest questions patients have is about side effects. What will it feel like when you start? Are they tolerable? How long do they last?

At Townhead Pharmacy, we support patients across Scotland on their weight loss journeys and help them understand what to expect. This guide will break down the common side effects of both treatments, compare their severity, and explain what typically happens in the first few weeks.

What Are Mounjaro and Wegovy?

Wegovy (semaglutide) is a GLP-1 receptor agonist. It works by mimicking a hormone that regulates appetite, slows digestion, and signals to your brain that you’re full sooner.

Mounjaro (tirzepatide) is a newer treatment that targets two hormones: GLP-1 and GIP. This dual action not only helps reduce appetite but also improves how the body manages insulin and fat storage.

Both medications are injected once a week and are prescription-only, typically recommended for adults with a BMI over 30, or over 27 with a weight-related condition such as type 2 diabetes or high blood pressure.

Common Side Effects: Mounjaro vs Wegovy

Here’s a breakdown of what patients most often report during the first weeks of treatment.

1. Nausea

Wegovy: Nausea is the most commonly reported side effect, especially as the dose increases. It tends to occur more frequently in the first 2–4 weeks and can feel mild to moderate.

Mounjaro: Also causes nausea, but some patients report it being milder or easier to manage than with Wegovy—possibly due to the dual hormone action that slows digestion less aggressively at lower doses.

What to do:

Eat smaller, slower meals. Avoid greasy or heavy foods. Stick to bland options like toast, rice, or crackers if nausea appears.

2. Constipation or Diarrhoea

Wegovy: Digestive changes like constipation are relatively common.

Mounjaro: Some users experience both constipation and diarrhoea, depending on individual response and diet.

What to do:

Stay hydrated, eat plenty of fibre, and keep physically active. If it becomes uncomfortable, our pharmacists can suggest safe remedies.

3. Fatigue or Low Energy

Wegovy: Some people experience tiredness, especially in the first week after starting or when the dose increases.

Mounjaro: May also cause fatigue, although it is less commonly reported than with Wegovy.

This is often a short-term response as your body adjusts to eating less and processing food more slowly.

4. Headaches

Both: Mild headaches can occur in the early stages of either treatment. It’s usually linked to dehydration, blood sugar changes, or shifts in eating patterns.

What to do:

Drink plenty of water and avoid skipping meals. Paracetamol can be used if necessary, but speak to your pharmacist if symptoms persist.

5. Loss of Appetite

While this may sound like a goal rather than a side effect, some patients experience very reduced appetite, which can lead to unintentional undereating or skipping meals entirely.

This is more noticeable with Mounjaro due to its stronger appetite-suppressing effect in some individuals.

What to do:

Focus on nutrient-dense meals, even in smaller portions, to ensure your body gets the fuel it needs.

Are These Side Effects Dangerous?

In most cases, no. Side effects tend to be mild to moderate, temporary, and manageable with simple lifestyle adjustments. The body typically adjusts within the first 4–6 weeks, especially if the dose is gradually increased as prescribed.

However, if you experience:

• –> Persistent vomiting

• –> Signs of dehydration

• –> Severe abdominal pain

• –> Difficulty tolerating food or fluids

You should contact our pharmacy team or your prescriber promptly.

Find the Best Treatment for You

When it comes to Mounjaro vs Wegovy, both offer effective, clinically tested paths to weight loss. The side effects are similar but may feel different from person to person. The most important thing is having the right support and realistic expectations from the start.

If you’re thinking about weight loss injections and have questions about side effects, speak to the Townhead Pharmacy team today. We’ll help you find the safest, most effective treatment—and walk with you every step of the way.

This blog was written on behalf of Townhead Pharmacy by Pharmacy Mentor.