Weight Loss Injections Side Effects

Weight loss injections like liraglutide, Wegovy, and Mounjaro can cause side effects for some people. In this guide, we’ll provide all the possible side effects of these medications and how best to deal with them. If you do get side effects, it is important that you speak to your doctor about it.

Brenda Ikeji

Medically reviewed by

Dr Brenda Ikeji

Last reviewed: 20 Jan 2025

What are the side effects of weight loss injections?

There are multiple potential side effects you can get from taking weight loss injections like liraglutide, Wegovy, and Mounjaro. However, not everyone gets side effects and how serious they are is different from person to person.

What are the most common side effects?

Very common side effects (may affect more than 1 in 10 people) of liraglutide, Wegovy, and Mounjaro include:

  • feeling sick (nausea)
  • being sick (vomiting)
  • diarrhoea
  • constipation
  • headache

Wegovy can also cause:

  • stomach pain
  • feelings of weakness or tiredness

Mounjaro can also cause:

  • stomach pain

These side effects are not usually serious, and you will normally get them for a few days or weeks before they go away.

If you take Mounjaro and get stomach (tummy) pain and/or constipation, these very common side effects may not decrease over time.

Common side effects (may affect up to 1 in 10 people) of liraglutide, Wegovy, and Mounjaro include:

  • indigestion
  • heartburn or acid reflux
  • stomach bloating
  • wind (flatulence)
  • belching
  • dizziness
  • injection-site reactions (itchiness or redness)

Liraglutide may also cause:

  • dry mouth
  • feelings of weakness and tiredness
  • difficulty sleeping in the first 3 months of treatment
  • rash on your skin
  • low blood sugar
  • increase in pancreatic enzymes, which can lead to pancreatitis
  • gastritis (inflammation of your stomach lining)
  • discomfort or pain in your stomach
  • changed sense of taste
  • gallstones

Wegovy may also cause:

  • change in skin sensation
  • hair loss
  • gastritis (inflammation of your stomach lining)
  • discomfort or pain in your stomach
  • changed sense of taste
  • gallstones

Mounjaro may also cause:

  • feelings of weakness and tiredness
  • allergic reactions such as rash, itching and eczema
  • low blood pressure
  • hair loss

What are the serious side effects of weight loss injections?

Common serious side effects (may affect up to 1 in 10 people) of Wegovy include:

  • complications of diabetic eye disease (diabetic retinopathy)

If you have diabetes and get changes in your vision whilst taking Wegovy, you should inform your doctor as soon as possible as if left untreated it may get worse.

Uncommon serious side effects (may affect up to 1 in 100 people) of liraglutide, Wegovy, and Mounjaro include:

  • inflamed pancreas (pancreatitis)

Wegovy can also uncommonly cause kidney or bladder stones.

Rare serious side effects (may affect up to 1 in 1,000 people) of liraglutide, Wegovy, and Mounjaro include:

  • severe allergic reactions such as angioedema or anaphylaxis

Wegovy can also rarely cause hip fractures.

Call 999 immediately if you or someone else gets signs of a severe allergic reaction. Symptoms may include:

  • sudden swelling of the face, lips, throat or mouth
  • difficulty breathing or breathing very fast
  • a fast heartbeat
  • a tight throat or struggling swallow
  • sudden confusion, drowsiness or dizziness
  • fainting and not waking up
  • for children, being floppy or limp and not responding like normal
  • a swollen, raised or itchy rash

A severe allergic reaction can be life-threatening and requires urgent hospital treatment.

Please note the possible side effects listed here for Mounjaro apply when it is used for weight loss. If it is used to treat type 2 diabetes the side effects are not exactly the same because the dose of the active ingredient is different.

How long do weight loss injection side effects last?

Weight loss injection side effects normally last a few days but can last up to a few weeks. They are usually mild and most commonly occur when you start taking the medication. This is because your body needs to get used to it. Side effects may also happen again when you increase your dose.

This is why weight loss injections have a dosing schedule and why your doctor might reduce your dose and extend the duration of your treatment to help you manage any side effects you get.

When do weight loss injection side effects start?

Side effects from weight loss injections normally start within the first few days of your first injection or after you increase your dose. However, they can happen at any time.

Can weight loss injections cause long-term side effects?

Most weight loss injection side effects are not long-term and can often be treated or will go away when you stop taking the medication.

For example, in a study where people took Mounjaro at the highest dose (15mg) for 72 weeks no significant negative long-term side effects were found. Researchers also found that participants had:

  • improved blood pressure
  • weight loss
  • lower blood sugar level if they had type 2 diabetes

Another study treating 205 obese and non-obese type 2 diabetics found that 20% of patients got side effects whilst taking liraglutide during the first 4 months but decreased to 2% of patients at 20 months. Long-term effects of taking liraglutide included:

  • weight loss
  • improved blood pressure
  • improved cholesterol

However, if you get side effects whilst taking weight loss injections you should notify your GP to make sure they are managed properly. Not addressing side effects may cause long-term health problems.

For example, if you have diabetes and get changes in your vision whilst taking Wegovy (or Ozempic), you should contact your GP as the medication may make it worse. Semaglutide, the active ingredient in Wegovy and Ozempic, has been linked to diabetic retinopathy which can cause blindness if left undiagnosed and untreated. If you get changes in your vision, you should report them as soon as possible so your doctor can decide the best course of action.

Do weight loss injections side effects get worse as I increase my dose?

When you take weight loss injections, the doctor will prescribe you with a dose which increases over time. This is part of the normal dosing schedule of these types of medications. Each time you increase your dosage, the risk of getting side effects is higher whilst your body gets used to the higher dose.

If you experience side effects that become severe or too uncomfortable, the doctor may reduce your dose or put you on a lower dose for a longer time to help you manage them.

How can I manage weight loss injection side effects?

Getting side effects doesn’t always mean weight loss injections are not suitable for you. Your body may get used to the medication and so the side effects will decrease over time. Many side effects are common and can be managed with other treatments or lifestyle changes.

Managing side effects at home can help you to continue taking weight loss injections and reach your weight loss goals.

Feeling sick (nausea) or being sick (vomiting)

Nausea and vomiting can be managed by:

  • drinking small amounts of liquids often
  • eating plain food in small portions
  • avoiding alcohol, sugary drinks and caffeine
  • not smoking, particularly on an empty stomach
  • taking your weight loss injection with food
  • introducing more food slowly as you start to feel better
  • fresh air and rest
  • anti-sickness tablets from a doctor or pharmacist

Repeated vomiting can make you dehydrated, so speak to a doctor if it doesn’t stop.

Diarrhoea

Diarrhoea can be managed by:

  • drinking lots of liquid, particularly water or electrolyte drinks
  • avoiding alcohol, fruit juice and caffeine
  • avoiding greasy, rich, fatty and spicy food
  • eating plain food in small portions as you start to feel better
  • getting plenty of rest

Persistent diarrhoea can cause dehydration. If you experience severe and long-lasting diarrhoea, speak to your doctor.

Constipation

Constipation can be managed by:

  • eating small, regular meals to avoid overfilling your stomach
  • a healthy balanced diet containing plenty of high-fibre food
  • food which can be a natural laxative, such as carrots, broccoli or beans
  • not consuming alcohol
  • increasing your fluid intake
  • over-the-counter medicines to soften your stool
  • using a small stool to raise your knees above your hips when you’re on the toilet

If your constipation persists for 3 days, speak to your doctor as you may need prescription-only medication to help relieve the symptoms.

Headache

A headache can often be managed by:

  • drinking plenty of water
  • avoiding alcohol
  • eating regular meals
  • resting, especially if you have the flu or a cold
  • taking painkillers such as paracetamol or ibuprofen
  • avoiding stress where possible
  • avoiding sleeping longer than normal
  • avoiding looking at a screen for a long time

If you experience prolonged bouts of headaches on Mounjaro, speak to your doctor as it could be a sign of something else other than side effects.

Stomach pain

Stomach pain can be managed by:

  • eating a healthy and balanced diet
  • eating small and regular meals instead of big ones
  • avoiding food that might be triggering stomach pain, such as rich, oily, greasy, acidic or spicy food
  • taking painkillers such as paracetamol or ibuprofen
  • a hot water bottle held against your stomach

If stomach pain persists or feels severe, it could be a sign of something more serious than side effects. Call 111, contact a doctor or go to A&E if you get severe stomach pain that doesn’t stop.

Feeling weak or tired

Feeling weak or tired can be managed by:

  • a healthy and balanced diet
  • staying hydrated
  • avoiding alcohol
  • regular exercise, which can increase your energy levels, although too much exercise can make you feel weak and tired
  • a regular sleep schedule of 8 or more hours per night
  • reducing caffeine intake, especially during the afternoon

If you continue to feel weak or tired, speak to your doctor. They may want you to take a blood test to check you are getting enough vitamins and minerals.

What should I do about serious side effects?

Serious side effects of weight loss injections will require advice and treatment from a doctor. They may tell you to stop taking the injections and provide you with treatment for the side effects.

Serious potential side effects of weight loss injections

These serious side effects can occur with liraglutide, Wegovy and Mounjaro. Please note that Wegovy has several more serious potential side effects, including:

Inflamed pancreas (pancreatitis)

This is an uncommon side effect. An inflamed pancreas (acute pancreatitis) can cause sudden severe stomach and back pain which does not go away.

Other symptoms include:

  • indigestion
  • feeling or being sick (vomiting)
  • a high temperature
  • a yellowing of the skin called jaundice, which may be less obvious on black or brown skin
  • yellowing of the whites of the eyes
  • tenderness or swelling of your tummy
  • rapid breathing
  • fast heartbeat

If you have acute pancreatitis, curling your body up or leaning forward might relieve the pain whereas lying on your back might make it worse. You might also feel worse very quickly after eating or drinking, particularly after fatty foods.

See a doctor immediately if you get symptoms of pancreatitis as it is a potentially life-threatening medical condition.

Severe allergic reaction

This is a rare side effect. Call 999 immediately if you or someone else gets signs of a severe allergic reaction. Symptoms may include:

  • sudden swelling of the face, lips, throat or mouth
  • difficulty breathing or breathing very fast
  • a fast heartbeat
  • a tight throat or struggling swallow
  • sudden confusion, drowsiness or dizziness
  • fainting and not waking up
  • for children, being floppy or limp and not responding like normal
  • a swollen, raised or itchy rash

A severe allergic reaction can be life-threatening and requires urgent hospital treatment.

Serious potential side effects of Wegovy

Wegovy has several other potential serious side effects which are not known to be caused by liraglutide or Mounjaro.

Diabetic retinopathy

This is a common serious side effect. If you have diabetes and get changes in your vision whilst taking Wegovy, you should inform your doctor as soon as possible, if left untreated it may get worse and eventually lead to blindness.

Kidney or bladder stones

This is an uncommon serious side effect of Wegovy. You may get pain in your lower tummy or back, find it hard to urinate or your urine might change colour. Seek medical assistance as soon as possible.

Hip fracture

This is a rare serious side effect. Symptoms of a hip fracture are normally:

  • severe pain in your hip or upper leg
  • not being able to put weight on the affected leg and hip
  • difficulty standing up
  • difficulty moving your hip or leg
  • a change in the angle or length of the affected leg

Seek medical attention immediately so you can get treatment.

Bowel obstruction

The frequency of this serious side effect of Wegovy is not known. It is a serious type of constipation which may cause symptoms such as:

  • stomach-ache
  • bloating
  • vomiting

This is a potentially life-threatening condition and you should seek hospital treatment immediately.

What makes weight loss injection side effects worse?

Several things may make side effects worse. Other medicines, the food and drink that you consume and dosing the weight loss injections properly are all important factors in how severe any side effects might be. Make sure you follow the dosing schedule exactly as prescribed by your doctor.

Other medicines

It is very important you tell your doctor about any other medicines you take, have recently taken or might take before starting weight loss injections. Not doing this could cause worse or more severe side effects.

What foods should I avoid on weight loss injections?

When you are taking weight loss injections, it is important to eat a healthy diet to help you to lose weight and avoid side effects. You should cut down significantly on certain food groups that are high in calories, cause blood sugar spikes or make side effects worse. You do not need to cut them out completely but consuming less of them should help make your weight loss journey easier.

These groups include:

  • ultra-processed foods
  • high-fat foods
  • carbonated drinks
  • refined carbohydrates like white pasta and white bread
  • caffeinated drinks and alcohol

Certain fruits and vegetables have a high glycaemic index (GI) rating which can cause side effects like blood sugar spikes, slow digestion and flatulence.

➤ Find out more about foods you should avoid on weight loss medications like Wegovy.

Can I drink alcohol on weight loss injections?

There is no clinical guidance against drinking alcohol whilst taking liraglutide, Wegovy or Mounjaro. Alcohol will not interact with the active ingredients in these weight loss medications.

However, excessive alcohol drinking might cause side effects or make existing side effects worse.

Many alcoholic drinks contain a lot of calories so consuming them might make losing weight more difficult. Drinking alcohol may also cause side effects which make it harder for you to stick to diet and exercise plans.

Will side effects go away if I stop taking weight loss injections?

Yes, most side effects will go away if you stop taking weight loss injections as the ingredients leave your body. However, some uncommon and serious side effects may be longer lasting and might need further treatment, such as pancreatitis or kidney stones.

How long do weight loss injections stay in your system?

How long weight loss injections stay in your system can vary according to the specific medication that you are taking. The dose you are taking will also affect how long the active ingredient stays in your system.

Approximate guidelines for liraglutide, Wegovy and Mounjaro are as follows:

Liraglutide

It takes about 3 days for liraglutide to leave your system. It has a half-life of 13 hours, meaning that 13 hours after taking it, 50% of the active ingredient is gone from your body.

Wegovy

It takes about 1 week from the last injection (of 2.4mg) for 50% of the active ingredient (semaglutide) to leave your body, and a total of 7 weeks for it to be gone completely.

Mounjaro

It takes about 25 days for Mounjaro to leave your body after the last injection. It has a half-life of 5 days of Mounjaro, meaning it will take 5 days for 50% of the active ingredient (tirzepatide) to leave your body.

How can I get weight loss injections out of my system faster?

It is not possible to speed up the process of the active ingredient leaving your body. Once you stop taking it, the ingredients begin to leave your body. If you are having difficulty with side effects, speak to a doctor about treatment options.

Frequently asked questions

Are weight loss injections safe?

Yes, weight loss injections are safe as long as they are prescribed by a doctor. Although there are many potential side effects, they are normally mild and will go away without any extra treatment. It is very rare that a patient gets serious side effects from weight loss injections.

Medically reviewed by

brenda-ikeji.png Dr Brenda Ikeji

Brenda studied medicine at St Georges University of London and has most recently worked in John Radcliffe Oxford University Hospitals, covering acute and general medicine.

Last reviewed: 20 Jan 2025

Sources

Saxenda, emc [accessed 31 December 2024]

Wegovy, emc [accessed 31 December 2024]

Mounjaro, emc [accessed 31 December 2024]

Constipation, NHS [accessed 31 December 2024]

Nausea, NHS [accessed 31 December 2024]

Headaches, NHS [accessed 31 December 2024]

Diarrhoea and vomiting, NHS [accessed 31 December 2024]

Stomach ache, NHS [accessed 31 December 2024]

Tiredness and fatigue, NHS [accessed 31 December 2024]

Broken hip, NHS [accessed 31 December 2024]

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