Mysimba Side Effects: Complete Safety Guide
When Do Side Effects Start? How Long Do They Last? Timeline, Contraindications & Food Interactions
Key Takeaways: Mysimba Side Effects
- Most common: Nausea, constipation, headache, dry mouth, dizziness, and trouble sleeping – these affect more than 1 in 10 people.
- When they start: Usually within the first 1‑2 weeks, especially during dose escalation.
- How long they last: Mild side effects often improve within 2‑4 weeks as your body adjusts.
- Serious side effects: Seizures (rare), suicidal thoughts, allergic reactions (angioedema), and severe skin reactions require immediate medical attention.
- Contraindications: Do not use if you have uncontrolled high blood pressure, seizure disorders, eating disorders, or take opioids/MAOIs.
- Food interactions: Take with food to reduce nausea; avoid alcohol; grapefruit may affect bupropion levels.
Mysimba is an effective weight‑loss medicine, but like all medications, it can cause side effects. Understanding what to expect, when side effects typically appear, and how long they last will help you manage them confidently. Always follow your doctor's advice and report any worrying symptoms promptly.
Seek Immediate Medical Help If:
You experience a seizure (fit), thoughts of harming yourself, swelling of the face/lips/tongue (angioedema), severe skin rash (possible Stevens‑Johnson syndrome), or signs of an allergic reaction. Also seek help if you have sudden vision changes or chest pain.
What Are Mysimba Side Effects?
Mysimba contains naltrexone and bupropion. Side effects can range from mild and temporary to serious but rare. Below we list the most common and important side effects based on clinical trials and the patient information leaflet.
Very common (affecting more than 1 in 10 people)
- Nausea – often the most frequent, especially in the first few weeks.
- Constipation – can be managed with diet and fluids.
- Headache – usually mild to moderate.
- Dry mouth – sip water regularly.
- Dizziness – avoid sudden movements.
- Insomnia (trouble sleeping) – take the evening dose earlier if needed.
Common (affecting up to 1 in 10 people)
- Vomiting, diarrhoea, abdominal pain
- Increased sweating, hot flushes
- Tremor, anxiety, feeling irritable
- Ringing in the ears (tinnitus)
- Blurred vision
- Increased blood pressure (your doctor will monitor this)
Serious side effects (rare – up to 1 in 1,000)
- Seizures (fits): Risk is about 1 in 1,000. Higher if you have risk factors.
- Suicidal thoughts or behaviour: Stop and contact a doctor immediately.
- Allergic reactions: Swelling of face, lips, tongue, throat – this is a medical emergency.
- Severe skin reactions: Stevens‑Johnson syndrome, erythema multiforme – rash with blistering or peeling.
- Liver injury: Yellowing of skin/eyes, dark urine.
🗒️ Note: This is not a full list. Always read the patient information leaflet and report any new or worsening symptoms to your doctor.
When Do Mysimba Side Effects Start?
Most side effects begin during the first 1‑2 weeks of treatment, particularly as your dose increases. The gradual dose escalation (over 4 weeks) is designed to help your body adjust and minimise side effects.
Typical onset times
- Nausea, headache, dizziness: Often appear within the first few days after starting a new dose.
- Constipation, dry mouth: May develop over the first week or two as the medicine builds up.
- Insomnia: Can start early; taking the evening dose with food and not too late may help.
- Increased blood pressure: Your doctor will check this before and during treatment; it can rise at any time.
- Serious side effects (seizures, allergic reactions): Can occur at any time, though seizures are more likely if you take too much or have risk factors.
If you start a new dose and feel worse, it's usually temporary. However, if symptoms are severe or worrying, contact your doctor.
How Long Do Mysimba Side Effects Last?
The duration of side effects varies from person to person. Most mild side effects improve within 2‑4 weeks as your body becomes used to the medicine. Some may persist longer but often become less bothersome.
Typical duration for common side effects
- Nausea: Usually improves within 1‑3 weeks. Taking tablets with food helps significantly.
- Headache and dizziness: Often resolve within the first month.
- Constipation: May persist but can be managed with increased fibre, fluids, and gentle exercise.
- Dry mouth: Can last throughout treatment – sucking sugar‑free sweets or sipping water helps.
- Insomnia: Sometimes improves as your sleep pattern adjusts; if not, your doctor may suggest taking the evening dose earlier.
If side effects continue beyond 4 weeks
Speak to your doctor. They may suggest ways to manage them, adjust your dose, or consider whether Mysimba is right for you. Do not stop the medicine without medical advice.
Mysimba Side Effects Timeline
The table below gives a general idea of when side effects may appear and how long they typically last. Everyone is different, so your experience may vary.
| Side Effect | When It Typically Starts | How Long It Usually Lasts |
|---|---|---|
| Nausea | First few days | 1‑3 weeks, then improves |
| Constipation | Within 1‑2 weeks | May persist, manageable |
| Headache | First week | Usually 2‑4 weeks |
| Dry mouth | First 1‑2 weeks | Often throughout treatment |
| Dizziness | Early in treatment | Usually 2‑4 weeks |
| Insomnia | First week | May improve over time; adjust timing |
| Increased blood pressure | Can occur at any time | Monitored by doctor; may need treatment |
| Seizures (rare) | Any time, especially with overdose | Emergency – stop medicine |
🗒️ Remember: If you experience any severe or persistent side effects, contact your doctor promptly.
Contraindications: Who Should NOT Take Mysimba
Mysimba is not suitable for everyone. Do not take Mysimba if any of the following apply to you:
Allergy
Allergic to naltrexone, bupropion, or any tablet ingredients.
Uncontrolled high blood pressure
Hypertension not well managed with medication.
Seizure disorders
Epilepsy or history of seizures (fits).
Eating disorders
Current or past bulimia or anorexia nervosa.
Opioid dependence
Current opioid use, opioid addiction, or acute withdrawal.
MAO inhibitors
Taking MAOIs (for depression or Parkinson’s) or within the last 14 days.
Severe liver disease
End‑stage liver disease or severe hepatic impairment.
End‑stage kidney disease
Severe renal impairment requiring dialysis.
Bipolar disorder
History of manic episodes or bipolar illness.
Brain tumour
Active brain tumour or CNS condition.
Pregnancy & breastfeeding
Do not use during pregnancy or while breastfeeding.
Alcohol or sedative withdrawal
If you have recently stopped heavy drinking or benzodiazepines.
Always provide your full medical history to your doctor before starting Mysimba.
Food Interactions: Alcohol, Grapefruit & More
What you eat and drink can affect how Mysimba works and your risk of side effects.
Take with food
Always take Mysimba with a meal (breakfast and evening meal). Food reduces nausea and helps your body tolerate the medicine. A high‑fat meal can increase peak levels slightly, but taking it consistently with food is most important.
Avoid alcohol
Drinking alcohol while on Mysimba increases the risk of seizures, dizziness, and may worsen side effects. If you are a heavy drinker, do not stop suddenly without medical advice – sudden withdrawal can also trigger seizures.
Grapefruit and grapefruit juice
Grapefruit can affect the liver enzyme (CYP3A4) that helps break down bupropion. This could lead to higher blood levels and increased side effects. It's best to avoid grapefruit while taking Mysimba.
Caffeine
Bupropion can make you feel more sensitive to caffeine. If you experience jitteriness or anxiety, consider reducing your caffeine intake (coffee, tea, energy drinks).
Other medicines and supplements
Always tell your doctor about any other medicines, herbal remedies, or supplements you take. Some (like St John's Wort) can interact with Mysimba.
Mysimba Side Effects FAQs
What is the most common side effect of Mysimba?
Nausea is the most frequent side effect, affecting more than 1 in 10 people. It usually improves within a few weeks, especially if you take the tablets with food.
Can Mysimba cause weight gain instead of loss?
No, Mysimba is designed to promote weight loss. However, some people may experience fluid retention or increased appetite as a side effect, but this is uncommon.
Should I stop Mysimba if I feel depressed?
If you experience new or worsening depression, or thoughts of self‑harm, stop the medicine and contact your doctor immediately. Do not restart without medical advice.
Can I take painkillers like codeine with Mysimba?
No. Mysimba contains naltrexone which blocks opioids, so codeine, morphine, and similar painkillers will not work. Avoid all opioids.
Does Mysimba affect blood pressure?
Yes, bupropion can increase blood pressure. Your doctor will check your BP before starting and regularly during treatment. If it rises significantly, treatment may need to be stopped.
Need Support with Weight Management?
If you're considering Mysimba and want to discuss side effects or your suitability, speak with a UK‑registered doctor through a confidential online consultation.
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Medical Content Manager
Nabeel is a co-founder and medical content manager of Chemist Doctor. He works closely with our medical team to ensure the information is accurate and up-to-date.
Medical Doctor
Dr. Feroz is a GMC-registered doctor and a medical reviewer at Chemist Doctor. He oversees acute condition and urgent care guidance.
Medical Director
Usman is a co-founder and medical director of Chemist Doctor. He leads the organisation's strategic vision, bridging clinical and operational priorities.
Review Date: 22 February 2026
Next Review: 22 August 2026
Published on: 22 February 2026
Last Updated: 22 February 2026