Common side effects of weight management medicines
Weight loss injections such as Mounjaro and Wegovy can be very helpful for suitable patients, but like all medicines, they can cause side effects.
Most side effects are linked to the digestive system. This is because these medicines affect appetite, fullness, stomach emptying and how the body responds to food. For many patients, side effects are mild and improve as the body adjusts. For others, symptoms can be more uncomfortable and may need extra support, a slower dose increase or a clinical review.
At Aster, we believe patients should know what to expect before starting treatment. Side effects are not something to panic about, but they should not be ignored either. The aim is to manage symptoms early, stay hydrated, eat enough and know when to ask for help.
Why side effects happen
Mounjaro and Wegovy work partly by helping you feel fuller for longer and reducing appetite. This can make it easier to eat smaller portions, but it can also slow digestion and change how your body responds to meals.
This is why some people feel sick, bloated, constipated, full very quickly or more sensitive to rich foods. These symptoms are often more noticeable when starting treatment or after moving up to a higher dose. The patient leaflets for both Mounjaro and Wegovy highlight digestive symptoms such as nausea, vomiting and diarrhoea, and Wegovy’s leaflet specifically warns that vomiting or diarrhoea can lead to dehydration if fluids are not replaced.
Nausea
Nausea is one of the most common side effects of weight loss injections. It may feel like mild queasiness, a heavy feeling after eating, or feeling sick when you smell or think about certain foods.
Nausea can be worse if you eat too quickly, eat large portions, eat very fatty foods or go for long periods without eating. Some people also feel more nauseous after a dose increase.
Things that may help:
Eat smaller portions
Eat slowly
Stop eating when you feel full
Avoid greasy, fried or very rich foods
Avoid lying down straight after eating
Sip fluids regularly
Try bland, lighter foods if needed
Avoid alcohol if it worsens symptoms
It can also help to avoid skipping food completely. An empty stomach can sometimes make nausea worse. If you cannot face a full meal, try something small but useful, such as Greek yoghurt, soup, eggs, toast with a protein source, or a small smoothie.
If nausea is persistent, severe or stopping you from eating and drinking, you should ask for clinical advice.
Vomiting
Vomiting can happen, especially when the body is adjusting to treatment or after a dose increase. Occasional vomiting may settle, but repeated vomiting should always be taken seriously because it can lead to dehydration.
If you vomit, focus on small sips of fluid. Do not try to eat a large meal straight away. Once you feel able to eat, start with small, plain foods.
Things that may help:
Sip water or an oral rehydration drink
Avoid large meals
Avoid rich or greasy foods
Eat slowly when you restart food
Rest and monitor symptoms
Ask for advice if vomiting continues
You should seek clinical advice if vomiting is repeated, you cannot keep fluids down, you feel dizzy or weak, or you have signs of dehydration such as dark urine, dry mouth or lightheadedness. The Wegovy leaflet specifically highlights the importance of drinking enough fluids to reduce dehydration risk during vomiting or diarrhoea.
Constipation
Constipation is very common during weight loss treatment. It can happen because the medicine slows digestion, but also because patients often eat less, drink less and reduce fibre without realising.
Constipation may feel like fewer bowel movements, harder stools, straining, bloating or discomfort.
Things that may help:
Drink fluids regularly
Increase fibre gradually
Eat vegetables, fruit, oats, beans, lentils or wholegrains
Include movement where possible
Avoid eating too little
Do not ignore the urge to go
Ask a pharmacist for advice if needed
Fibre is helpful, but it needs fluid to work properly. Increasing fibre without drinking enough can sometimes make constipation worse.
You should ask for advice if constipation becomes painful, lasts several days, causes significant bloating, or is associated with vomiting or abdominal pain.
Diarrhoea
Some patients experience diarrhoea, particularly when starting treatment or increasing the dose. This may settle as the body adjusts, but it can still be uncomfortable and can increase the risk of dehydration.
Things that may help:
Sip fluids regularly
Consider oral rehydration if needed
Avoid alcohol
Avoid greasy or very spicy foods
Eat smaller, simpler meals
Monitor how often it is happening
Ask for advice if symptoms continue
Do not ignore diarrhoea if it is severe, persistent, contains blood, or is associated with fever, weakness or dehydration. Patients with kidney problems or those taking medicines affected by dehydration should be especially cautious.
Reflux, indigestion and burping
Some people notice reflux, indigestion, heartburn or increased burping. This can happen because food may stay in the stomach for longer, and larger or richer meals can sit more heavily.
Things that may help:
Eat smaller meals
Avoid eating late at night
Avoid lying down soon after eating
Reduce fried, fatty or spicy foods
Reduce alcohol and fizzy drinks if they trigger symptoms
Eat more slowly
Raise the head of the bed if night-time reflux is an issue
If reflux is frequent, painful or not improving, speak to a pharmacist or clinician. Do not keep relying on over-the-counter medicines without checking whether your treatment plan needs reviewing.
Appetite suppression that feels too strong
Reduced appetite is expected, but appetite suppression can sometimes feel too strong. Some patients find they are barely eating, skipping meals or feeling repelled by food.
This is not always a good thing. Eating too little can increase the risk of tiredness, dizziness, nausea, constipation, poor protein intake and muscle loss.
Signs your appetite may be too suppressed include:
Struggling to finish very small meals
Frequently skipping food all day
Feeling weak or lightheaded
Not managing protein intake
Nausea when trying to eat
Rapid weight loss that feels difficult to control
Constipation or dehydration
If this happens, the answer is not to push through silently. Your dose may need reviewing. Sometimes it is better to stay on a lower dose for longer rather than increase too quickly.
Headache and tiredness
Some patients report headaches, tiredness or feeling low in energy. This may be linked to eating less, dehydration, changes in blood sugar, poor sleep or the body adjusting to treatment.
Things that may help:
Drink enough fluids
Eat regular small meals
Include protein
Avoid very low calorie intake
Get enough sleep
Reduce alcohol
Review caffeine intake
Check blood glucose if you have diabetes and have been advised to monitor
If tiredness is severe, persistent or unusual for you, ask for advice.
Low blood sugar in some patients
Weight loss injections do not usually cause low blood sugar on their own in people without diabetes. However, the risk can be higher in patients with diabetes who use insulin or certain tablets such as sulfonylureas. Wegovy’s patient information notes that it must not be used as a substitute for insulin and highlights low blood sugar warnings for relevant patients.
Symptoms of low blood sugar may include sweating, shaking, hunger, confusion, dizziness, palpitations or feeling faint.
If you have diabetes, your clinician should review your medication and advise you on monitoring. Do not change diabetes medication without professional advice.
When side effects need urgent advice
Most side effects are mild to moderate, but some symptoms need urgent medical attention.
Seek urgent help if you develop:
Severe abdominal pain that does not go away
Pain spreading to the back
Persistent vomiting
Signs of dehydration
Symptoms of a severe allergic reaction
Fainting or severe weakness
Blood in vomit or stool
Severe diarrhoea
Yellowing of the skin or eyes
Severe right upper abdominal pain
Symptoms of low blood sugar that do not resolve
Severe, persistent abdominal pain can be a warning sign of pancreatitis. This is listed in patient information for these medicines, and the UK regulator has also warned about a small but serious risk of acute pancreatitis with GLP-1 medicines.
Do not rush dose increases
Many side effects happen during dose escalation. This is why dose reviews matter.
A higher dose is not automatically better. If you are losing weight, tolerating treatment and feeling well, it may not always be necessary to move up quickly. If you are having significant side effects, increasing the dose may make them worse.
A clinical review should look at:
Weight change
Appetite control
Side effects
Hydration
Bowel habits
Protein intake
Energy levels
Current medicines
Medical history
Whether the dose is still suitable
The right dose is the one that is safe, tolerated and effective for you.
The Aster approach
At Aster, we support patients throughout treatment because side effects are easier to manage when they are reported early. We want patients to feel comfortable telling us what is happening, even if symptoms feel embarrassing or minor.
Weight loss treatment should not leave you feeling unsupported. Nausea, constipation, reflux, diarrhoea and appetite changes are common, but they still need proper advice. In some cases, small changes to meals, fluids and dose timing may help. In other cases, treatment may need reviewing.
The aim is not just to lose weight. The aim is to lose weight safely, while protecting your health, nutrition and long-term wellbeing.
This article is for general information only and does not replace medical advice. Weight loss medicines are prescription-only and may not be suitable for everyone. A clinician should assess your individual circumstances before treatment is started or continued.
This article was written by Sally
Operations Manager
Aster, Pharmacy Clinic Edinburgh & Arcadia Health Clinic
