Side Effects of Weight Loss Injections: What to Expect and How to Manage Them

If you are researching Mounjaro or Wegovy, side effects are probably one of the first things you have searched for. That is entirely reasonable. Before starting any medication, you want to know what you are getting into and what to do if things feel uncomfortable.

The good news is that the side effects of GLP-1 weight loss injections are well understood, well documented, and for the vast majority of patients, manageable. They are also largely temporary. This article gives you an honest, clinician-led overview of what to expect, why side effects happen, how to reduce their impact, and when it is worth getting in touch with your doctor.

Why do GLP-1 weight loss injections cause side effects?

GLP-1 medications work partly by slowing gastric emptying, the rate at which food moves from your stomach into your digestive system. This is the same mechanism that makes them effective for weight loss: food stays in your stomach longer, you feel fuller sooner, and hunger signals reduce. But during the initial adjustment period, this slowing of digestion can cause your gut to protest a little.

The most common side effects, primarily nausea and other gastrointestinal symptoms, are essentially your digestive system adjusting to a new hormonal signal it has not encountered before. For most patients they are a sign that the medication is working, not that something has gone wrong.

"One of the most common concerns patients have before starting GLP-1 treatment is the possibility of side effects. The good news is that when side effects do occur, they are usually mild, temporary, and most often affect the digestive system. Symptoms such as nausea, bloating, constipation, diarrhoea, or mild indigestion are generally most noticeable when starting treatment or increasing the dose.

In my experience, these symptoms often improve as the body adapts. Taking a gradual approach to dose increases, staying well hydrated, eating smaller meals, and having access to clinical support can make a significant difference. It's important for patients to remember that they don't have to manage these symptoms alone. With the right guidance, most people are able to continue treatment successfully and achieve meaningful improvements in their health."

— Dr Saira Bano, GP with a special interest in metabolic health and obesity at SwiftDoctor

The most common side effects of Mounjaro

Mounjaro (tirzepatide) has a well-characterised side effect profile from extensive clinical trial data across the SURMOUNT and SURPASS programmes, involving more than 9,000 participants. The most reported side effects are gastrointestinal and are typically mild to moderate in severity. [1]

The side effects most likely to occur, particularly during the first weeks of treatment or following a dose adjustment, include nausea, diarrhoea, constipation, vomiting, stomach pain, reduced appetite, and fatigue. Across clinical trials, nausea was reported by around 24% of participants and diarrhoea by 12 to 22%, with most events rated as mild to moderate. [1]

Nausea

Nausea is the most commonly reported side effect of Mounjaro and tends to be most noticeable in the early weeks of treatment or around the time of a dose adjustment. It usually improves within a few days as your body adapts.

Practical steps to manage nausea include eating smaller, more frequent meals; avoiding rich, fatty, or heavily spiced food around the time of your injection; staying well hydrated; and injecting at a consistent time each week. Taking your injection in the evening when you can rest afterwards can also help.

Stomach pain and gastrointestinal issues

Bloating, stomach discomfort, constipation, and diarrhoea are all commonly reported, particularly during the adjustment period. For constipation, increasing fibre intake gradually, staying well hydrated, and gentle movement can all help. For diarrhoea, avoiding overly rich or high-fat foods tends to reduce symptom frequency. Most gastrointestinal symptoms are self-limiting and improve as your body adjusts.

If you experience severe or persistent abdominal pain, particularly pain that radiates to your back, contact your clinician promptly, as this can occasionally indicate pancreatitis. [2]

Fatigue

Some patients report feeling more tired than usual, particularly in the early weeks of treatment. Prioritising sleep, eating nutritious meals even when appetite is reduced, and avoiding dramatic food restriction beyond what the medication naturally prompts all help. Fatigue typically improves as the body adjusts.

The most common side effects of Wegovy

Wegovy (semaglutide) shares a broadly similar gastrointestinal side effect profile to Mounjaro, given that both work through GLP-1 receptor activation. In the STEP 1 clinical trial, nausea was reported in approximately 44% of participants on the full 2.4 mg dose, a higher rate than seen with tirzepatide in SURMOUNT-1, though direct comparisons require caution as the trials used different populations and escalation protocols. [3] Constipation was reported in around 24% of participants. [3]

The practical management guidance for nausea, gastrointestinal symptoms, and fatigue is the same as described for Mounjaro above. At SwiftDoctor, doses are only increased when a patient has reached a plateau in their weight loss and needs a higher dose to continue progressing.

Nausea in particular deserves a specific note for Wegovy patients. The 44% rate reported in STEP 1 is meaningfully higher than the roughly 24% seen with Mounjaro, and patients switching from Mounjaro to Wegovy or starting Wegovy as a first treatment should be prepared for this difference. That said, the same management strategies apply: smaller meals, lighter foods, consistent injection timing, and good hydration. Most patients find nausea peaks and then settles once a stable dose is reached. For the minority in whom it persists, it is worth discussing with your clinician whether temporarily pausing escalation might help.

Hair loss

Some patients notice increased hair shedding during treatment with GLP-1 medications. It is important to understand that the medication itself is not directly damaging hair follicles. The hair thinning is a condition called telogen effluvium, a temporary, reversible form of shedding triggered by the physiological stress of rapid weight loss, reduced calorie intake, or nutritional changes, rather than by the drug itself. [4]

In the STEP 1 trial, hair loss was reported in 2.5% of semaglutide participants versus 1% on placebo, with higher rates seen in those who lost more weight more quickly, supporting the theory that it is the weight loss driving the shedding rather than the medication. [4] A 2026 systematic review confirmed that rapid weight loss is the primary contributing factor for telogen effluvium in this context. [5]

Hair typically begins to regrow once weight stabilises and nutrition is optimised. The most important practical steps are ensuring adequate protein intake throughout treatment, eating a balanced diet rich in vitamins and minerals, and staying hydrated. If you are concerned about nutritional deficiencies during treatment, SwiftDoctor's nutrition blood test can help identify any gaps that may be contributing.

When to contact your doctor about side effects

Most side effects resolve on their own, but you should seek prompt clinical advice if you experience severe or persistent abdominal pain, particularly if it radiates to your back or is accompanied by vomiting, as this may indicate pancreatitis. Persistent vomiting that prevents you keeping food or fluids down also requires attention. Any side effect that is worsening rather than improving after several weeks is worth discussing with your clinician.

SwiftDoctor provides ongoing clinical support throughout treatment. If something does not feel right, get in touch with our team promptly. 

Are weight loss injections safe long term?

Both Mounjaro and Wegovy are licensed prescription treatments reviewed and approved by the MHRA. The long-term cardiovascular safety profile of semaglutide is particularly well established, with the SELECT trial demonstrating a significant reduction in major cardiovascular events. [6] Long-term data on tirzepatide continues to accumulate, with three-year SURMOUNT data showing sustained weight loss and a favourable safety profile. [1]

Post-marketing surveillance continues to refine the safety picture over time. The most important safeguard is that treatment is prescribed and monitored by a qualified clinician rather than obtained through unregulated channels.

Frequently Asked Questions

Can side effects be a sign the medication is not right for me? 

Not necessarily. Most side effects, particularly nausea and gastrointestinal symptoms, are a normal part of the adjustment period rather than a signal that the medication is unsuitable. They tend to improve significantly within a few weeks. If side effects are severe, persistent, or worsening, that is the point at which a clinical conversation is warranted rather than stopping treatment independently.

Is hair loss permanent on GLP-1 medications? 

No. The hair shedding associated with GLP-1 treatment is almost always temporary and linked to the physiological stress of rapid weight loss rather than the medication itself. Most patients see regrowth once their weight stabilises and nutritional intake is optimised. Ensuring adequate protein and micronutrient intake during treatment is the most effective preventive step.

Do Mounjaro and Wegovy cause the same side effects? 

Broadly yes, since both work through GLP-1 receptor activation. Nausea and gastrointestinal symptoms are the most common for both. Constipation tends to be reported somewhat more frequently with Wegovy, while the overall side effect profiles are similar enough that the same management strategies apply to both.

Can I take anything to manage nausea on GLP-1 injections? 

Over-the-counter remedies such as ginger products or anti-nausea medications may provide some relief, but it is worth discussing with your clinician before adding anything new. Dietary adjustments, consistent injection timing, and staying hydrated tend to be the most effective first-line strategies and should be tried before reaching for additional medication.

What is the difference between a side effect and a serious adverse reaction? 

Side effects are expected, generally mild responses to a medication that most patients experience to some degree. A serious adverse reaction is uncommon, more severe, and requires medical attention. For GLP-1 medications, the main serious adverse reaction to be aware of is pancreatitis, signalled by severe abdominal pain radiating to the back. Any symptom that feels significantly different from mild discomfort warrants prompt clinical contact.

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Sources

1. Jastreboff, A.M. et al. (2022). Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine, 387, 205–216. https://doi.org/10.1056/NEJMoa2206038

2. Electronic Medicines Compendium (2024). Mounjaro 2.5 mg solution for injection: Summary of Product Characteristics. Eli Lilly. https://www.medicines.org.uk/emc/product/15484/smpc

3. Wilding, J.P.H. et al. (2021). Once-weekly semaglutide in adults with overweight or obesity (STEP 1). New England Journal of Medicine, 384, 989–1002. https://doi.org/10.1056/NEJMoa2032183

4. Gupta, A.K. et al. (2026). GLP-1 therapies and hair loss: a systematic review of current evidence and implications for counseling. International Journal of Surgery, 122. https://doi.org/10.1177/00368504261444578

5. Revolution Health & Wellness (2025). Hair loss with GLP-1 medications: telogen effluvium explained. https://revolutionhealth.org/blogs/news/hair-loss-glp1-medications

6. Lincoff, A.M. et al. (2023). Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). New England Journal of Medicine, 389, 2221–2232. https://doi.org/10.1056/NEJMoa2307563