Weight-Loss Injection Side Effects Explained

GLP-1 weight-loss injections are clinically approved treatments used to support weight loss in people who struggle to lose weight through lifestyle changes alone. Medications such as Wegovy (semaglutide) and Mounjaro (tirzepatide) work by regulating appetite, slowing digestion, and improving blood-sugar control, which helps reduce calorie intake and support long-term weight loss when combined with diet and physical activity [1][2].

As with all prescription medicines, weight-loss injections can cause side effects. Most side effects are mild to moderate, occur early in treatment, and improve as the body adapts to the medication [3]. Understanding potential side effects helps patients know what to expect and when to seek medical advice.

For a broader overview of how GLP-1 weight-loss injections work, who they are suitable for, and what to expect from treatment overall, see our GLP-1 weight loss injections complete guide.

What Side Effects Can Weight Loss Injections Cause?

The majority of side effects associated with GLP-1 weight-loss injections affect the digestive system. This is due to how these medications slow gastric emptying and act on appetite-regulating hormones in the brain [3][4].

Side effects are most commonly reported:

• When starting treatment

• During dose increases

• If meals are large, high in fat, or eaten quickly

Common Side Effects

The most frequently reported side effects include:

• Nausea

• Vomiting

• Diarrhoea

• Constipation

• Abdominal discomfort or bloating

• Reduced appetite

• Feeling full sooner than usual

Clinical trial data for both semaglutide and tirzepatide shows gastrointestinal side effects are most noticeable in the early weeks of treatment and typically reduce over time as patients reach a maintenance dose [1][2][3].

Less Common Side Effects

Some people may experience additional side effects, although these are reported less frequently:

• Headache

• Fatigue or low energy

• Dizziness

• Acid reflux or indigestion

• Injection-site reactions such as redness, itching, or mild swelling

Injection-site reactions are usually mild and resolve without treatment [1][2].

Rare but Serious Side Effects

Serious side effects are uncommon, but it is important to be aware of them.

Seek urgent medical advice if you experience:

• Severe or persistent abdominal pain

• Symptoms of pancreatitis, such as severe upper abdominal pain that may radiate to the back

• Signs of gallbladder disease, including right-sided abdominal pain or jaundice

• Persistent vomiting that may lead to dehydration

GLP-1 medications are not suitable for everyone, and a full medical assessment is required before starting treatment [5].

Why Do Side Effects Occur?

GLP-1 receptor agonists mimic a naturally occurring hormone that:

• Slows digestion

• Reduces hunger signals

• Enhances feelings of fullness

These effects support weight loss but can temporarily disrupt normal digestive processes, particularly during dose escalation. This is why doses are increased gradually to improve tolerability [3][4].

How to Reduce Side Effects

Many side effects can be reduced with simple changes:

• Eat smaller, more frequent meals

• Avoid high-fat or greasy foods

• Eat slowly and mindfully

• Stay well hydrated

• Avoid lying down immediately after eating

• Follow the prescribed dosing schedule

Most patients find side effects improve significantly once they reach a stable dose [3].

Food choices can have a significant impact on how well side effects are tolerated. Our guide on what to eat while using weight loss injections explains which foods are easier to digest and how diet can help minimise nausea and other digestive symptoms during treatment.

When to Speak to a Clinician

You should speak to a healthcare professional if:

• Side effects interfere with daily life

• Symptoms worsen rather than improve

• You experience severe abdominal pain or ongoing vomiting

Ongoing clinical monitoring helps ensure treatment remains safe and effective [5].