What are the side effects of weight loss injections?

In recent years, weight loss injections have become one of the most talked about treatments for managing excess weight in the UK. For many people, they can support significant and sustainable weight loss when combined with a reduced calorie diet and regular exercise. 

These medications work by mimicking natural gut hormones to reduce appetite, slow digestion and keep you feeling fuller for longer. But like any prescription medication, they can cause side effects.

At Pharmulous, you receive a consultation with a UK registered clinician before we prescribe anything, so we can assess your suitability and medical history before treatment begins. Start your weight loss consultation to find out which treatment may be suitable for you.

Do weight loss injections have side effects?

Yes, weight loss injections cause side effects for some people, particularly in the early stages of treatment or after each dose increase. As your body adjusts, many of these effects ease off, though it can take a few weeks for each new dose to settle.

Most side effects are mild, predictable and temporary. A small number of people experience more serious reactions that need prompt medical attention - but knowing what to expect and when to seek advice helps to ensure you get the support you need. 

Important! The information given below is general and for illustrative purposes only. Speak to your prescriber for tailored advice relating to the specific medication you’re planning to take, and to understand how your medical history may affect your experiences. Some side effects may be more or less common depending on your reason for taking the medication (e.g. for weight management as opposed to type 2 diabetes).

Common side effects

These side effects may be experienced by more than 1 in 10 people: 

• Nausea or vomiting
• Diarrhoea or constipation
• Stomach pain
• Headache
• Weakness or fatigue
• Changes to vision, in those with diabetes
• Low blood sugar, if taken alongside certain other diabetes medicines.

These are usually manageable with simple measures, such as eating smaller meals, avoiding rich or fatty food and staying well hydrated. 

Less common side effects

These side effects are rarer:

• Gallstones
• Dizziness or low blood pressure
• Indigestion and/or bloating
• Reflux and/or heartburn
• Hair loss
• Increased heartrate
• Changes to skin tone (jaundice)
• Fatigue
• Kidney or bladder stones
• Changes in taste perception or skin sensitivity
• Injection site reactions, such as redness or itching.

If you experience irritation at the injection site, rotating locations (e.g. thigh, upper arm, abdomen) may help.

Rare but serious side effects

These symptoms are much less common, but require urgent medical attention:

• Pancreatitis, severe abdominal pain that may spread to your back, alongside nausea or vomiting 
• A severe allergic reaction, such as swelling of the face, lips or throat, or difficulty breathing (affecting less than 0.1% of people)
• Serious loss of vision (affecting less than 0.01% of people).

If you experience any of the rare but serious symptoms above, stop treatment and call 999 or attend A&E.

Weight loss injections containing tirzepatide (usually branded as Mounjaro in the UK) may also reduce the effectiveness of oral contraceptives, so you should use an alternative method of contraception during your treatment.

While it can be alarming to read about potential side effects, it’s worth knowing that your consultation helps to reduce the risk of these effects happening. Our clinicians assess your situation to ensure any medications prescribed are suitable for you, and you’ll be given advice on how to manage and minimise any side effects during treatment. 

Got questions? Get in touch with our friendly staff for more advice.

Which weight loss injection has the least side effects?

There is no single weight loss injection proven to have the least side effects. Wegovy and Mounjaro all cause similar gastrointestinal side effects in clinical trials, because they all work in more or less the same way. 

How well you tolerate each medication varies from person to person. Clinical trials haven’t found a clear winner when it comes to tolerability.

What does make a measurable difference is how treatment is managed. To reduce the risk of side effects you should:

• Start on the lowest available dose
• Increase the dosage gradually and under clinical supervision
• Avoid stopping, starting or changing your treatment without medical advice.

Most of the time, side effects are nothing to worry about when taking weight loss medications, but they shouldn’t be ignored. If your symptoms are severe, persistent or getting worse, speak to a trained medical professional for personalised advice.