Common side effects explained

Understanding side effects

Side effects are a normal part of starting any new medication. With GLP-1 medications, most side effects relate to the digestive system — a direct result of how the medication works (slowing gastric emptying and affecting gut hormones).

Most side effects are mild, occur early in treatment, and improve within a few weeks.

Digestive side effects (very common)

Nausea

The most frequently reported side effect. Usually mild to moderate, most common in the first 4–8 weeks and after each dose increase. See our dedicated Managing nausea guide for strategies.

Constipation

Because food moves through your system more slowly, constipation is common. Strategies that help:

• Increase fluid intake — aim for 2+ litres of water daily
• Eat more fibre: vegetables, legumes, wholegrains
• Stay physically active — even a daily walk helps gut motility
• If constipation is persistent, talk to your pharmacist about appropriate options

Diarrhoea

Less common than constipation, but some people experience loose stools, particularly early in treatment. Usually resolves on its own. Staying hydrated is important.

Bloating and gas

A side effect of slower gastric emptying. Eating smaller meals, avoiding carbonated drinks, and reducing foods that cause gas (cabbage, beans, onions) can help.

Vomiting

Less common than nausea. If vomiting is severe or persistent, contact your prescriber.

Heartburn / acid reflux

Slowed stomach emptying can worsen reflux in some people. Eating smaller meals, avoiding lying down after eating, and reducing acidic or spicy foods can help.

Injection site reactions (common)

Mild redness, bruising, or itching at the injection site is normal and usually resolves within a day or two. Rotating injection sites helps prevent localised reactions. If you notice a hard lump or significant swelling at a site, discuss with your prescriber.

Fatigue (less common)

Some people feel tired in the first few weeks. This often improves as your body adjusts. Ensure you are eating enough — significantly reduced calorie intake can contribute to fatigue.

Headaches (less common)

Headaches in the early weeks are usually related to dehydration or reduced calorie intake rather than a direct drug effect. Ensure you are drinking enough water and eating regular small meals.

Side effects that are uncommon but important to know about

Gallstones

Rapid weight loss (from any cause) can increase the risk of gallstones. Symptoms include severe pain in the upper right abdomen, particularly after eating fatty foods. Contact your prescriber if you experience this.

Pancreatitis (rare)

Severe, persistent abdominal pain that radiates to the back, accompanied by nausea and vomiting, could indicate pancreatitis. This is rare but serious. Seek medical attention promptly if you experience these symptoms.

Thyroid concerns (rare)

Some GLP-1 medications carry a precautionary note about thyroid tumours based on animal studies. If you have a personal or family history of thyroid cancer or certain thyroid conditions, discuss this with your prescriber before starting treatment.

Side effects checklist: when to contact your prescriber

Contact your prescriber if you experience:

• Severe or persistent vomiting
• Inability to keep fluids down
• Severe abdominal pain
• Signs of an allergic reaction (rash, swelling, difficulty breathing)
• Symptoms of low blood sugar (shakiness, sweating, confusion) — more relevant if you also take other diabetes medications
• Any new or concerning symptoms you are unsure about

General information note

This guide provides general educational information only — not medical advice. Side effect experiences vary between individuals. Always discuss any concerns with your prescriber or pharmacist.