Most Common Side Effects of Injectable Weight Loss Medications and How to Manage Them

Injectable weight loss medications such as Mounjaro and Wegovy have revolutionised obesity treatment, offering significant weight loss benefits for eligible patients. However, like all medications, these treatments can cause side effects, particularly during the initial weeks of treatment.

At Happy Pharmacy, we understand that starting Weight Loss Medication can feel overwhelming, especially when you're worried about potential side effects. The good news is that most side effects are mild and temporary, usually improving within the first few weeks as your body adjusts to the medication.

This comprehensive guide outlines the most common side effects you may experience and provides practical, easy-to-follow management strategies to help you continue your weight loss journey safely and comfortably.

If you haven't started yet and are still thinking about options, why not come and take our online Weight Loss Consultation to see which treatment may be right for you.

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Understanding Injectable Weight Loss Medications

Injectable weight loss medications work by mimicking hormones naturally produced in your body. These clever treatments slow gastric emptying, reduce appetite, and help regulate blood sugar levels, all contributing to sustainable weight loss that you can maintain long-term.

• Mounjaro (tirzepatide) targets both GLP-1 and GIP receptors.

• Wegovy (semaglutide) is a GLP-1 receptor agonist.

Clinical trials have demonstrated remarkable results, with Mounjaro showing up to 22.5% weight loss over 72 weeks at the maximum maintenance dose – that's life-changing results for many people.¹

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Very Common Side Effects (Affecting More Than 1 in 10 People)

1. Nausea and Vomiting

Why it occurs:
The delayed gastric emptying caused by GLP-1 receptor activation creates feelings of fullness and can trigger nausea, particularly when starting treatment or increasing doses. Think of it as your body learning to work with the medication.

Simple management strategies that really work:

• Stay well-hydrated by drinking at least 8 cups of water daily

• Eat small, regular meals to avoid an empty stomach

• Choose bland, low-fat foods and avoid greasy or spicy meals

• Try soothing beverages such as ginger, peppermint, or fennel teas

• Consider anti-sickness medications after consulting your pharmacist

When to seek assistance:
Contact NHS 111 if vomiting occurs more than three times daily or if you experience signs of dehydration, including dark urine or dizziness.

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2. Diarrhoea

Why it occurs:
As your digestive system adapts to hormonal changes, bowel movements may become more frequent or loose, particularly during the first few weeks of treatment. This is completely normal and usually temporary.

Proven management strategies:

• Drink plenty of clear fluids, including water or diluted squash

• Use oral rehydration salts available from your pharmacy

• Eat small, bland meals and avoid caffeine, alcohol, and fatty foods

• Consider over-the-counter loperamide for 1–2 days maximum

• Rest and avoid over-exertion

• Gradually reintroduce foods as symptoms improve

When to seek assistance:
Persistent diarrhoea, presence of blood in stools, or inability to stay hydrated requires immediate contact with NHS 111.

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3. Constipation

Why it occurs:
Slower digestion caused by delayed gastric emptying can lead to reduced bowel movement frequency. Your digestive system is simply taking more time to process food.

Effective management strategies:

• Maintain adequate hydration with at least 8 cups of water daily

• Include high-fibre foods such as prunes, figs, and oats in your diet

• Increase physical activity to stimulate bowel movements

• Consider mild laxatives available over-the-counter

• Try positioning aids such as a footstool whilst on the toilet

When to seek assistance:
If management strategies prove ineffective after several days, or if you notice blood in your stool, consult your GP or NHS 111.

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4. Headaches

Why it occurs:
Headaches may result from dehydration, changes in eating patterns, or blood sugar fluctuations associated with reduced food intake.

Management strategies:

• Ensure adequate hydration throughout the day

• Maintain regular eating patterns, even with smaller portions

• Rest in a quiet, dark environment

• Take simple painkillers such as paracetamol as directed

• Apply cold compresses to your forehead or temples

When to seek assistance:
Persistent or worsening headaches despite management efforts warrant consultation with your GP or NHS 111.

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Common Side Effects (Affecting Up to 1 in 10 People)

5. Heartburn and Acid Reflux

Why it occurs:
Delayed gastric emptying can increase stomach acid exposure to the oesophagus, causing burning sensations and discomfort.

Management strategies:

• Eat smaller, more frequent meals

• Avoid trigger foods including spicy, fatty, or acidic items

• Sleep with your head slightly elevated

• Avoid lying down immediately after eating

• Consider over-the-counter treatments such as Gaviscon or proton pump inhibitors like omeprazole

When to seek assistance:
Contact your GP if symptoms persist despite treatment, or if you experience difficulty swallowing or vomiting blood.

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6. Fatigue and Tiredness

Why it occurs:
Reduced caloric intake, lifestyle changes, or other side effects such as nausea may contribute to feelings of tiredness.

Management strategies:

• Maintain a balanced diet with adequate nutrients

• Consider multivitamin supplements if dietary intake is insufficient

• Ensure adequate hydration

• Include regular, gentle exercise such as walking

• Maintain consistent sleep patterns with at least 8 hours nightly

• Limit screen time before bedtime and avoid caffeine after midday

When to seek assistance:
Persistent fatigue may indicate nutritional deficiencies and should be discussed with your healthcare provider.

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7. Injection Site Reactions

Why it occurs:
Local reactions including redness, swelling, itching, or pain may occur at injection sites, typically resolving as your body adjusts to treatment.

Management strategies:

• Rotate injection sites weekly between different areas

• Ensure proper injection technique with clean hands and sterile equipment

• Apply ice packs before injection to reduce discomfort

• Use antihistamine creams or tablets for itching

• Keep injection sites clean and dry

When to seek assistance:
If reactions worsen, spread beyond the injection site, or become unbearable, discontinue treatment and consult your GP immediately.

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8. Dizziness

Why it occurs:
Blood pressure changes, dehydration, or low blood sugar levels may contribute to dizziness, particularly in patients with diabetes.

Management strategies:

• Stay well-hydrated throughout the day

• Move slowly when changing positions

• Avoid driving or operating machinery when dizzy

• Take injections after eating to reduce post-injection dizziness

• Manage stress levels through relaxation techniques

When to seek assistance:
Frequent dizziness, especially with other symptoms such as weakness or confusion, requires medical evaluation.

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Serious Side Effects of Mounjaro and Wegovy

While the side effects we've discussed so far are common and manageable, there are some serious side effects that, although rare, require immediate medical attention. It's important to be aware of these so you know when to seek urgent help. Remember, these serious side effects are uncommon, but being informed helps you stay safe during your treatment.

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Gallstones and Gallbladder Inflammation

Frequency: Affects up to 1 in 10 Wegovy users; up to 1 in 100 Mounjaro users.

Prevention strategies:

• Maintain steady weight loss of no more than 1.5kg (3 pounds) per week

• Stay hydrated and eat smaller, frequent meals

• Follow a balanced, moderate-fat diet

When to seek assistance:
Severe pain under the ribs, persistent vomiting, fever, or jaundice require immediate medical attention via NHS 111 or A&E. Don't try to manage severe gallbladder symptoms at home – these conditions need professional medical treatment.

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Dehydration

Symptoms: Dark-coloured urine, feeling light-headed or dizzy, inability to keep fluids down, passing urine less often than usual, dry mouth and skin, or feeling confused.

When to seek assistance:
If you can't keep fluids down for more than 24 hours, or if you notice multiple signs of dehydration, contact NHS 111 or visit A&E. Severe dehydration can affect your kidney function and requires medical intervention.

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Pancreatitis

Symptoms: Severe abdominal pain radiating to the back, accompanied by nausea and vomiting. The pain is usually intense and doesn't improve with normal painkillers.

When to seek assistance:
This rare but serious condition requires urgent medical attention. If you experience severe, persistent abdominal pain that radiates to your back, especially if accompanied by nausea and vomiting, contact NHS 111 or attend A&E immediately. Don't wait to see if it gets better on its own.

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Severe Allergic Reactions (Anaphylaxis)

Symptoms: Difficulty breathing, facial swelling, severe skin reactions, dizziness, or rapid pulse. You might also experience widespread itching or a feeling of impending doom.

When to seek assistance:
If you experience any signs of a severe allergic reaction, call 999 immediately. This is a medical emergency that requires immediate professional treatment. Take your medication packet with you to the hospital.

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Managing Side Effects Through Dosage Adjustments

We may recommend dosage modifications based on side effect severity:

• Mild side effects: Continue current dose for an additional week before increasing

• Moderate side effects: Reduce dose temporarily, then increase gradually

• Severe side effects: Stop medication immediately and seek medical advice

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Long-term Considerations

Most side effects are temporary and diminish as your body adjusts to treatment. Studies of Mounjaro at maximum maintenance doses for 72 weeks showed no negative long-term effects, with participants experiencing benefits including reduced blood pressure and improved glucose control.²

However, some conditions require ongoing monitoring:

• Dehydration from persistent vomiting or diarrhoea can affect kidney function

• Acute pancreatitis may recur in approximately 25% of affected individuals³

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When to Contact Happy Pharmacy

Our friendly clinical team is here to support you every step of the way. We're available to assist with any concerns about side effects, no matter how small they might seem. Don't hesitate to reach out – we're here to help make your weight loss journey as smooth as possible.

Seek medical advice immediately if you experience:

• Persistent vomiting or diarrhoea lasting more than 48 hours

• Signs of dehydration (dark urine, dizziness, dry mouth)

• Severe abdominal pain

• Allergic reactions (swelling, rash, difficulty breathing)

• Any symptoms causing significant concern

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Conclusion

Injectable weight loss medications offer significant benefits for eligible patients, and whilst side effects can occur, they're usually manageable and temporary. The key is knowing what to expect and having the right support in place.

At Happy Pharmacy, we believe that everyone deserves access to effective weight loss treatments with expert guidance every step of the way. Our experienced clinical team doesn't just prescribe your medication – we're your partners in achieving lasting weight loss success.

Most of our patients find that any initial side effects are a small price to pay for the life-changing results Mounjaro and Wegovy can deliver. With proper management, clinical support, and a bit of patience, you can successfully navigate the early weeks of treatment and focus on achieving your weight loss goals.

Your weight loss journey starts here. Whether you're just considering treatment or already taking medication, we're committed to providing the personalised support you need to succeed.

For expert advice, contact us at help@happypharmacy.co.uk – because your health and success matter to us.

 

Blog medically reviewed by : Palvinder Deol, GPhC Registered Pharmacist, 22 May 2025

References

1. Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387(3):205–216.

2. Dahl D, Onishi Y, Norwood P, et al. Effect of subcutaneous tirzepatide vs placebo added to titrated insulin glargine on glycaemic control in patients with type 2 diabetes: the SURPASS-5 randomised clinical trial. JAMA. 2022;327(6):534–545.

3. Rawla P, Sunkara T, Gaduputi V. Epidemiology of gallbladder cancer. Clin Exp Hepatol. 2019;5(2):93–102.

4. National Health Service. Semaglutide (Wegovy) for weight loss [Internet]. London: NHS; 2023 [cited 2024 Oct 09]. Available from: https://www.nhs.uk

5. Medicines and Healthcare products Regulatory Agency. Mounjaro patient information leaflet [Internet]. London: MHRA; 2023 [cited 2024 Oct 09]. Available from: https://www.gov.uk/mhra