If you're taking or thinking about starting a GLP-1 medication for weight loss, it helps to know what side effects are common, which ones need urgent attention, and what monitoring you should expect. This guide covers the general picture. It does not replace advice from your GP, pharmacist, or prescriber, who can look at your own health history.
GLP-1 medications include semaglutide (brand name Wegovy, available as an injection and a tablet), tirzepatide (brand name Mounjaro), liraglutide (brand name Saxenda), and orforglipron (brand name Foundayo). They work by slowing digestion and reducing appetite.
This guide is general information, not personal medical advice. If you're worried about a symptom, contact your GP, pharmacist, or prescriber. If you think you're having a medical emergency, call 999 or go to A&E.
Common side effects
Most people who take a GLP-1 medication get some digestive side effects, especially in the first few weeks or after a dose increase. Commonly reported side effects include, in no particular order:
- Nausea
- Diarrhoea
- Constipation
- Vomiting
- Stomach pain or bloating
- Reduced appetite
- Injection site reactions (redness, itching, or a small lump)
- Headache
- Fatigue
For many people, these side effects reduce over the first few weeks. For others, they continue for longer or don't improve. Your prescriber or pharmacist can suggest ways to manage mild side effects like nausea or constipation. If side effects don't ease, or they're affecting your daily life, speak to your prescriber. Any decision to stop or change your dose should be made with your prescriber.
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Serious side effects: when to seek help
Serious side effects are less common, but you should know what to look out for.
- Signs of dehydration from ongoing vomiting or diarrhoea: dizziness, dark urine, or passing very little urine
- Changes in mood that are new or worse than usual
- Worsening of existing diabetic eye problems, if you have diabetes and are taking semaglutide
- Symptoms of low blood sugar (shakiness, sweating, confusion), particularly if you also take insulin or other diabetes medication
- Signs of a severe allergic reaction: swelling of your face, lips, tongue, or throat, difficulty breathing, or a fast-spreading rash
- Severe, constant stomach pain that spreads to your back, with or without vomiting, this can be a sign of pancreatitis and needs urgent attention
- Sudden pain in the upper right side of your stomach, yellowing of your skin or eyes, dark urine, or pale stools
- Sudden vision loss or a sudden change in vision. This is a rare but recognised risk with semaglutide (Wegovy, Ozempic)
Some people have reported mood changes, including suicidal thoughts, while taking GLP-1 medications. UK regulators (the MHRA) reviewed this and concluded that current evidence does not support a causal link between GLP-1 medications and depression or suicidal thoughts, though they continue to monitor it. Even so, tell your prescriber about any new or worsening changes in mood. If you ever have thoughts of harming yourself, treat it as something to raise with a healthcare professional without delay. If you are in crisis, you can call 111, contact the Samaritans on 116 123, or go to A&E.
Interactions to be aware of
Tell your prescriber and pharmacist about every medicine, supplement, and over-the-counter product you take. GLP-1 medications can interact with:
- Insulin and other diabetes medication: taking these together with a GLP-1 medication can cause low blood sugar. Your diabetes medication doses may need to be reviewed.
- Oral contraceptives (if you're taking tirzepatide/Mounjaro): tirzepatide can reduce how well the contraceptive pill is absorbed. If you take the pill and are starting or increasing your dose of tirzepatide, ask your prescriber or pharmacist whether you need additional contraceptive precautions, and for how long. This advice is specific to tirzepatide, it does not currently apply to semaglutide (Wegovy, Ozempic).
- Other medications taken by mouth: slowed digestion can change how quickly other oral medicines are absorbed. Flag any regular medication to your prescriber before starting.
- Alcohol: no direct dangerous interaction is established, but alcohol can worsen nausea and stomach side effects, and may affect blood sugar if you have diabetes.
This is not a full list. Always check with a pharmacist or your prescriber, particularly if you're on multiple medications.
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Monitoring requirements
What monitoring you get, and how often, depends on whether you're prescribed through the NHS or privately, and on your individual health.
Monitoring may include some combination of, depending on your provider and individual health:
- Weight and BMI checks
- Blood pressure
- Blood tests (kidney function, liver function, blood glucose or HbA1c if you have diabetes)
- Review of side effects and tolerability at each dose increase
- Annual or ongoing review while you continue treatment
NHS vs private: monitoring and access at a glance
| NHS pathway | Private pathway | |
|---|---|---|
| Initial assessment | Semaglutide (Wegovy): through a specialist weight management service. Tirzepatide (Mounjaro): can also be started in primary care, subject to eligibility criteria | Consultation with a private prescriber or clinic, usually online or in person |
| Eligibility threshold | Set by NICE guidance, generally BMI 35+ with a weight-related health condition, or a lower BMI in some cases. Thresholds are adjusted for some ethnic groups. NHS-funded rollout is currently phased and may be narrower than the NICE threshold in your area | Provider-specific criteria, within the medicine's licence, which can be broader than NHS-funded thresholds |
| Typical wait time | Can involve a wait for specialist referral, this varies by area and by which medication you're eligible for | May be quicker after a private consultation, but this varies by provider |
| Ongoing monitoring | Built into the structured NHS programme, follow-up appointments included | Varies by provider, check what's included before you start |
| Approximate cost | Free at the point of use if eligible (standard NHS prescription charge applies in England) | Prices vary widely by provider, medication, and dose, and change over time. Check the total cost, including consultations, before you start |
| Continuity of care | Integrated with your GP record | May not be shared with your GP unless you tell them |
Whichever route you use, tell your GP that you're taking a GLP-1 medication, even if it was prescribed privately. This keeps your NHS medical record accurate and safe, particularly if you need other treatment or surgery.
Frequently asked questions
Sources
- MHRA. MHRA finds evidence does not support a link between GLP-1 receptor agonists and suicidal and self-injurious thoughts and actions, 4 September 2024.
- MHRA. GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis, including necrotising and fatal cases, Drug Safety Update, 29 January 2026.
- MHRA. MHRA updates guidance for semaglutide prescribers and patients, 5 February 2026.
- MHRA. GLP-1 medicines for weight loss and diabetes: what you need to know.
- Faculty of Sexual and Reproductive Healthcare (FSRH). CEU statement: glucagon-like peptide-1 (GLP-1) agonists and oral contraception, February 2025.
- NICE. Semaglutide for managing overweight and obesity, technology appraisal guidance TA875.
- NICE. Tirzepatide for managing overweight and obesity, technology appraisal guidance TA1026.
- Electronic Medicines Compendium (emc). Wegovy (semaglutide): summary of product characteristics.
- Electronic Medicines Compendium (emc). Ozempic (semaglutide): summary of product characteristics.
- Electronic Medicines Compendium (emc). Mounjaro (tirzepatide): summary of product characteristics.