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Side Effects

Learn more about the side effects associated with Wegovy and Mounjaro

Common Side Effects

Like most medications, Wegovy and Mounjaro can cause side effects, particularly as their dosage increases.

The following lists contain some of the most common side effects reported during the clinical trials*

Please note that the suggested remedies are general recommendations found on NHS websites.

If your symptoms are persistent or severe, please contact a GP or medical professional for advice.

*Wilding, J.P.H. et al. (2021) “Once-weekly semaglutide in adults with overweight or obesity,” New England Journal of Medicine, 384, pp. 989–1002

*Jasterboff, A.M. et al. (2022) “Tirzepatide Once Weekly for the Treatment of Obesity” New England Journal of Medicine, 387, pp.205-216.


Nausea and Bloating

Participants on both Wegovy and Mounjaro commonly experienced mild to moderate nausea, particularly when participants stepped up the strength of their pen.

Nausea was less common with Mounjaro than with Wegovy.

It often goes away as you continue using the drug, but some helpful tips to deal with nausea include:

  • Getting plenty of fresh air
  • Drinking ginger or peppermint tea
  • Eating smaller meals more frequently
  • Sipping cold water throughout the day to avoid dehydration


Diarrhoea can be an uncomfortable symptom when starting your treatment. However, many people have reported mild symptoms that have subsided over time. Here are a few ways you can manage diarrhoea:

  • Drinking plenty of liquids, including water or juice
  • Avoid foods that can make your diarrhoea worse, including spicy or fatty foods
  • Reduce your caffeine intake
  • Stick to light, bland foods


Some participants also report constipation from the drug.

Constipation is usually mild and does not persist for long periods of time.  Helpful, simple remedies include:

  • Drinking more fluids: aim for 6–8 glasses per day
  • Adding fibre to your diet. Good sources of fibre include whole grains, nuts, and seeds
  • Increase your physical activity where possible
  • Establish a regular bathroom routine and do not delay your urge to use the toilet


Vomiting was also reported in clinical trials.  Again, this tends to occur as you step up to higher dose pens, or in patients with poor eating habits (eating too much and too quickly despite lacking an appetite). Here are a few helpful tips to manage vomiting:

  • Eat foods that have a high water content, such as cucumbers and soups
  • Reduce your portion sizes and eat smaller meals more frequently; overeating is a common cause of vomiting when taking weight loss injections
  • Slow down when eating
  • Do not eat just for the sake of it when you don’t feel hungry but do make sure you have at least two small meals a day
  • Take regular sips of a cold drink
  • Eat light, bland foods

Headache or Dizziness

Headaches have been reported in 15% of those on the Wegovy treatment and it is also seen with Mounjaro.

Usually mild, you can manage these symptoms by:

  • Place a hot or cold compress on your forehead
  • Simple painkillers such as paracetamol. It is best to avoid anti-inflammatory drugs like ibuprofen and voltarol
  • Get plenty of rest in a quiet or dark room
  • If you feel dizzy: lie down until the dizziness passes
  • Avoid coffee, smoking and alcohol

Indigestion and Heartburn

Indigestion and heartburn were also associated with taking the injections.

To help manage  acid reflux

  • Adjust your sleeping position by raising your pillow or the head of your bed
  • Avoid late meals and eat right before going to bed
  • Identify trigger foods that may increase your heartburn (such as spicy food, wine, onions)
  • Avoid smoking

Efficacy of the contraceptive pill (Mounjaro)

When oral contraceptives were taken with Mounjaro, the levels of certain hormones in the contraceptives were lowered.

This means we can’t be sure if the oral contraceptive pill will work as well when you are on Mounjaro, so it’s suggested that users switch to an alternative method of birth control.

Hair Loss (Mounjaro)

4% of participants in the Mounjaro clinical trial experienced hair loss or thinning. Most of the cases were mild, and the majority of patients got better while still on the treatment. None of the patients who took Mounjaro stopped taking it or left the study because of hair loss.

Other Side Effects

This list is not intended to be a comprehensive list of all possible side effects that have ever been reported with treatment.

A useful guide compiled by Wegovy’s manufacturer, Novo Nordisk, can be found here

Another guide compiled by Mounjaro’s manufacturer, Eli Lilly, can be found here

Side Effects

In clinical trials, treatment caused side effects preventing normal activities in less than 4% of patients on Wegovy and 0.1% of patients on Mounjaro (compared with those taking a placebo dummy injection). However, there are some rare, but potentially more severe, side effects you should be aware of.  Most of these are just as likely to occur with Wegovy as with Mounjaro.

If you experience any of the side effects listed below, stop your treatment immediately and seek medical attention.

Pancreatitis -1 in 500 risk

Pancreatitis is an inflammation of the pancreas gland (part of the digestive system). It is often mild and self-limiting but can become serious if left untreated. If you experience sharp, persistent pain in the centre of your stomach and that extends to your back, stop using the pens immediately and seek medical attention.

Gallbladder- Related Disorders – about 1 in 100 risk

Weight loss injections may cause gallbladder-related disorders, including gallstones (cholelithiasis). Common symptoms of gallstones include persistent, severe stomach pains under the right rib cage, often with a high fever, and sometimes yellowing of the whites of the eyes (known as jaundice). If you experience any of these side effects, please stop taking the medication immediately and seek medical attention.

Injection site reactions (1 in 20 risk on Mounjaro) and Allergic Reactions

While reactions such as some swelling, tenderness, redness, or itching around the injection site are fairly common, a very serious allergic reaction to the pen is rare.  However, if you have any symptoms of an allergic reaction, such as severe rashes and itching, swelling of the tongue, face, throat, or lips, and difficulty breathing or swallowing, stop using the pens immediately and seek urgent medical attention.

For diabetics – Low blood sugar (hypoglycaemia)  – 1 in 100 risk

If you have diabetes or are taking certain diabetes medications such as insulin or another weight loss injection, taking the pens may lead to low blood sugar. We will ask supplementary questions about your diabetes treatments to assess your suitability for the treatment during the SemaPen programme onboarding process.  It is important to make sure to measure your blood sugar levels regularly and speak to your diabetes healthcare professional if you experience any symptoms of hypoglycaemia such as dizziness, anxiety and palpitations, sweating, turning pale or fainting.

Gastroparesis – Rare

All GLP 1 agonists like Wegovy and Moujaro work (in part) by slowing down stomach emptying to promote a feeling of early and more sustained fullness.  Gastroparesis is a very rare condition reported with GLP1 agonists, which is a particularly severe manifestation of this expected effect of treatment.  In gastroparesis, the muscles lining the stomach slow down to such a marked extent that undigested food consumed hours or, in more serious cases, days before can sit around in the stomach and be vomited back. If you are concerned that you are experiencing gastroparesis or vomiting frequently, you should contact your clinical support team for medical professional advice.

Mental health disturbance – Rare

Following a small number of case reports in Iceland and the USA, agencies that monitor the safety of new medicines are assessing a potential link between the use of weight loss injections and thoughts of self-harm.  At the time of writing, no causal association has been demonstrated, but our advice is to stop your treatment and seek urgent help from your family doctor or mental health practitioner in the unlikely event that you experience suicidal thoughts whilst on the treatment.  In addition, we will send you questionnaires to monitor your mental health during your time on the SemaPen programme.

Changing your Dose if you Experience Side Effects

In some cases, particularly as you start taking higher doses, you may find that you experience increased side effects. This is completely normal and is the reason why we increase your dose gradually. If you are unable to tolerate the side effects of your new dose, we recommend switching back to your previous dose.

If you are unable to tolerate the side effects of your new dose

We recommend switching back to your previous dose and resuming the SemaPen programme at a dose you are comfortable with. This doesn’t mean you will not lose any more weight, but you may find that your rate of weight loss can slow down. You may still see results on the lower dose.

If you want to reduce your dosage

You can reduce your dosage and switch back to your previous month’s pen strength by simply logging into your SemaPen account using the SemaPen app and changing your subscription. Please note that you will need to do this a few days before your next billing, to allow sufficient time for processing.

If you feel satisfied with your current dose

If you are satisfied with your current dose and do not wish to automatically step up to a higher strength pen next month, you can edit your plan details through the SemaPen app. Please note that you will need to do this every month, as our program will automatically upgrade your dose each month.

Have another question?

You can read answers to frequently
asked questions here.