Guide to Weight Loss Injections Side Effects
For many people who want to lose weight, lifestyle changes alone can feel frustratingly slow or ineffective, and for some, they’re not possible due to other health conditions. In recent years, prescription weight loss injections like Wegovy® (semaglutide) and Mounjaro® (tirzepatide) have transformed how doctors and qualified weight management professionals can help people with their weight loss journey. These medications offer clinically proven results when combined with a healthy diet and regular exercise.
As with any medication, weight loss injections can sometimes cause side effects, especially in the early stages of treatment as the body adjusts. This guide aims to help you understand what to expect, why these side effects occur, how to manage them, and when to speak to a healthcare professional.
Why do weight loss injections cause side effects?
Most side effects from medications like Wegovy® and Mounjaro® are a result of how they work in the body.
They are part of a class of medicines known as GLP-1 receptor agonists (and in the case of Mounjaro, a dual GIP/GLP-1 agonist). These medications work by mimicking natural hormones that regulate appetite and digestion.
They help:
Reduce hunger and food cravings (“food noise”)
Increase feelings of fullness after eating
Slow down digestion, helping you feel fuller for longer
Support blood sugar control, which can reduce unhealthy snacking or overeating
Because they slow down digestion and alter how your body handles glucose and appetite signals, these medicines naturally impact your digestive system and metabolism.
This means that your body often needs time to adjust, especially in the first few weeks. Side effects are usually mild to moderate and often temporary, usually resolving as your system gets used to the medication.
Common side effects of weight loss injections
Most people who take Wegovy® or Mounjaro® experience some mild side effects early in treatment. These can include:
Digestive side effects of weight loss injections
Nausea – the most common side effect, often temporary
Vomiting – may occur if the dose is increased too quickly
Diarrhoea or constipation
Abdominal pain or bloating
Indigestion or heartburn
These symptoms happen because GLP-1 and GIP hormones slow down stomach emptying as part of how the medicine helps control appetite.
Other possible side effects of weight loss jabs
Fatigue or tiredness
Mild headaches
Dizziness (especially if eating less)
Injection site reactions, such as redness or itching
While these can be inconvenient, most people find they become less frequent or disappear altogether after the first few weeks.
Tips for managing weight loss injection side effects
We’ve compiled some practical ways to help minimise and manage side effects while your body adjusts to the weight loss medication:
Follow the prescribed dosage carefully: Increasing doses too quickly can worsen side effects.
Eat smaller meals: Large or fatty meals can increase feelings of nausea. Opt for smaller, balanced meals throughout the day and stop eating when you feel full.
Avoid greasy or spicy foods: These can sometimes trigger stomach discomfort if eaten when taking weight loss injections.
Stay hydrated: Sip water regularly, especially if you’re feeling nauseous or experiencing diarrhoea.
Eat slowly: Taking your time when eating helps prevent overeating and can reduce nausea.
Try ginger or peppermint if you feel nauseous: Ginger tea, peppermint tea, or mints can soothe mild nausea for some people.
Rest when needed: Mild fatigue or dizziness often improves as your calorie intake stabilises.
Test different injection sites: A recent clinical study found that the likelihood of side effects were higher depending on where you inject Mounjaro. This included:
| Injection site | Reported % of participants with side effects |
|---|---|
| Abdomen | ~ 68.5 % |
| Upper arm | ~ 57.4 % |
| Thigh | ~ 43.4 % |
If you’re experiencing side effects that are severe or very persistent, don’t try to push through without support! Your prescriber can help adjust your dose or timing, which can often reduce side effects.
Are weight loss injections safe?
When prescribed appropriately and used under medical supervision, Wegovy® and Mounjaro® are considered safe and effective treatments for weight management.
Both medications have undergone extensive clinical trials and are approved in the UK for specific use in adults with obesity or who are overweight with related health risks.
However, as with all prescription medicines, they are not suitable for everyone. You should not take weight loss injections if you:
Have a personal or family history of certain rare thyroid cancers
Have severe digestive disorders such as gastroparesis
Are pregnant or planning pregnancy
Have had an allergic reaction in the past to semaglutide, tirzepatide, or any ingredient in the medicine.
Your prescriber will carry out a medical assessment before issuing a prescription to help ensure it’s safe and suitable for you.
When to seek medical advice about weight loss jab side effects
You should contact your prescriber or a healthcare professional if you’re taking weight loss injections and you experience:
Persistent vomiting or inability to keep fluids down
Severe abdominal pain, especially with vomiting
Yellowing of the skin or eyes (jaundice)
Swelling or severe allergic reaction around the injection site
Signs of dehydration (such as dizziness, dry mouth, or dark urine)
In rare cases, side effects may indicate a more serious issue such as pancreatitis (inflammation of the pancreas), so always seek prompt medical help if symptoms seem unusual or severe.
Do weight loss jab side effects go away on their own?
For most people, side effects lessen or disappear completely after the first few weeks or months of taking weight loss injections. If your side effects continue, your prescriber may recommend steps such as:
Staying on your current dose for longer before increasing
Adjusting the timing of your injection
If you decide to transition off weight loss injections, side effects typically fade quickly, though your appetite and digestion will also return to normal, and weight regain may occur without continued lifestyle changes.
FAQs: Weight loss injection side effects
-
Everyone can respond differently to medications and weight loss injections are no different. Some people find Wegovy® easier to tolerate, while others experience fewer side effects with Mounjaro®.
Mounjaro® targets two hormones (GLP-1 and GIP), which may mean different digestive effects. Generally, both have similar side effect profiles, with nausea being the most common for both. -
Most side effects, especially nausea or digestive changes, will usually improve within a few days after stopping your normal dose. However, your appetite will likely return, so it’s important to plan ahead with your prescriber to maintain healthy habits and prevent rapid weight regain.
-
Moderate alcohol consumption is not strictly prohibited if you’re taking weight loss injections, but alcohol can worsen some side effects, such as nausea and affect blood sugar control, especially in the early stages. It’s generally recommended to limit your alcohol intake and monitor how your body responds in the first few months of taking the medication.
-
Not necessarily. Many people tolerate dose increases without major discomfort, especially if they follow a phased dosage increase plan. If you experience some nausea after a planned dose increase, it usually settles after a few days.
-
Some food choices can help with managing nausea caused by taking weight loss jabs. You can opt for bland, low-fat foods such as toast, rice, bananas, or soups. Avoid large meals, fried foods, and anything overly rich or spicy. Eating slowly and staying hydrated often helps too.
-
Serious side effects when taking weight loss injections are rare, but they can occur. One condition is pancreatitis, which usually presents as severe abdominal pain and vomiting. Allergic reactions to the ingredients of a weight loss injection are also uncommon but possible. Always report anything unusual to your prescriber.
-
Mild side effects like nausea or constipation usually last a few days to a few weeks. Once your body adjusts to the dose, these effects typically fade.
While side effects are relatively common, they are usually mild, temporary, and manageable with the right support and self-care strategies. The key is to start slowly, follow medical guidance, and listen to your body as it adapts to the medication.
If you ever experience side effects that feel unusual or last longer than expected, contact your prescriber or GP for advice.
With the right approach, many people find that the benefits of taking weight loss injections far outweigh the minor and short-lived side effects, and that these medications can play a transformative role in improving long-term health and weight management.
Find out if you are eligible for weight loss injections
