Chapter 5 of 11

Side effects

Why side effects happen, what to expect, what helps for each one, and when to seek advice. Most are mild and temporary.

Starting a GLP-1 medication is a significant step, and it’s completely natural to have questions about sideeffects. The most important thing to understand is that many of these effects are manageable, and often temporary as your body adapts.

This chapter will help you understand what to expect, why side effects can happen, and what you can do to feel more comfortable and supported.

Why do side effects happen?

GLP-1 medications work by slowing digestion and changing the way your body regulates appetite and fullness. Food stays in your stomach for longer, which helps you feel full sooner and for longer. This is a key reason these medications are so effective, but it can also lead to temporary digestive symptoms as your body adjusts. These effects are not harmful — in fact, they are often a sign that the medication is working.

The process

  1. You eat
  2. Food stays in your stomach longer
  3. You feel full sooner
  4. You eat less over time
  5. Supports weight loss & metabolic health

When are side effects most noticeable?

  • Early weeks (1–2)Side effects are most noticeable
  • After dose increasesSymptoms may briefly return
  • After a few weeksYour body adapts and symptoms may improve

Nausea and vomiting

Nausea is one of the most common early side effects. It occurs because food remains in the stomach for longer, which can feel unfamiliar at first. It is often triggered by eating too quickly, eating large portions, or choosing foods that feel too heavy.

What can help:

  • Eat small meals more frequently
  • Choose bland foods such as toast, yoghurt, or soup
  • Eat slowly and chew well
  • Sip fluids regularly
  • Opt for cold foods, which may feel easier to tolerate
  • Avoid rich, fatty or spicy foods
  • Try ginger or peppermint tea

If vomiting is severe (three or more times a day) or you are unable to keep fluids down for several hours, you should seek medical advice urgently.

Constipation

Slower digestion, combined with reduced food and fluid intake, can lead to constipation, especially in the early days.

What can help:

  • Increasing fluid intake
  • Gradually increasing fibre intake (e.g. oats, vegetables, fruit)
  • Maintaining regular movement and gentle activity
  • Consider a fibre supplement or an osmotic laxative if needed

Fatigue & headaches

Feeling tired in the early stages is common, often due to reduced calorie intake, insufficient hydration and adjustments in blood sugar. As GLP-1 medication can reduce your sense of thirst as well as hunger, aim to drink 2–3 litres of water a day to support your body and minimise side effects.

Stomach pain — when to take it seriously

Mild stomach pain can occur as food sits in the digestive system for longer than usual. Very rarely, GLP-1 medications have been associated with more serious conditions such as pancreatitis or gallstones, usually linked to rapid weight loss rather than the medication itself. It's important not to try to self-diagnose.

Less common or rare side effects include:

  • Hair loss or thinning
  • Injection site reactions
  • Flatulence
  • Changes in taste
  • Dizziness
  • Low blood pressure

Seek urgent medical attention (via NHS 111, A&E or 999) if you experience severe or persistent vomiting, signs of dehydration, severe abdominal pain, or symptoms of an allergic reaction. These situations are extremely rare.

A balanced perspective

While side effects are relatively common, they are normally manageable and tend to improve with time. You are not expected to 'push through' discomfort alone. If something doesn't feel right, reach out early — support is always available.