Before You Start: Setting Expectations
Starting a GLP-1 medication is a significant step. Whether you have been prescribed Mounjaro (tirzepatide), Wegovy (semaglutide), or Saxenda (liraglutide), the first month involves your body adjusting to a new medication. Understanding what is normal can reduce anxiety and help you stick with the treatment.
All three medications start at a low dose that gradually increases over several weeks. This titration period is designed to let your body adjust and minimise side effects. Your prescriber will provide a schedule specific to your medication.
Week 1: Your First Injection
The first injection is often the most nerve-wracking, but the pre-filled pens used by Mounjaro and Wegovy are designed to be straightforward. Most people inject into the abdomen, thigh, or upper arm.
In the first few days, you may notice a mild reduction in appetite. Some patients report feeling slightly nauseous, particularly after eating large or rich meals. This is the GLP-1 mechanism beginning to work — slowing your stomach emptying and signalling fullness to your brain.
Eat smaller, more frequent meals during the first week. Avoid greasy or heavy foods, and stay well hydrated. Many prescribers recommend eating slowly and stopping as soon as you feel satisfied.
Week 2: Appetite Changes Become Noticeable
By the second week, most patients notice a meaningful reduction in appetite and food noise — the constant background thinking about food that many people with obesity experience. This is one of the most commonly reported benefits.
Weight loss in the first two weeks is typically 1–2kg, though this varies widely. Some of this initial loss is water weight. Consistent, sustainable fat loss develops over the following weeks and months.
Weeks 3–4: Finding Your Rhythm
By weeks three and four, side effects usually begin to settle. Your body is adjusting to the medication, and you may find it easier to make healthier food choices simply because your appetite has reduced.
Clinical trial data from both SURMOUNT-1 and STEP 1 shows that weight loss is gradual and cumulative. The greatest changes come over months, not weeks. The first month is about building habits and letting the medication work alongside dietary and lifestyle changes.
When to Contact Your Prescriber
Most side effects are mild and temporary. However, you should contact your prescriber, GP, or NHS 111 if you experience:
- Severe or persistent vomiting that prevents you from keeping fluids down
- Severe abdominal pain that does not improve (possible sign of pancreatitis)
- Signs of an allergic reaction such as swelling, rash, or difficulty breathing
- Any thoughts of self-harm or significant mood changes
- Signs of dehydration such as dark urine, dizziness, or confusion
For serious symptoms including severe abdominal pain, allergic reactions, or thoughts of self-harm, seek immediate medical attention or call 999.