Mysimba Side Effects: What to Expect & Timeline
Mysimba(naltrexone/bupropion) is a weight loss medication that may cause side effects ranging from mild nausea to serious seizures. This guide explains all potential side effects, when they typically start, and how long they last.
Key Takeaways
- Most common: Nausea (affects >1 in 10), headache, constipation, insomnia
- When they start: Gastrointestinal effects begin within days; neurological effects may take 2-4 weeks
- How long they last: Most subside within 4-6 weeks as your body adjusts
- Serious risks: Seizures (1 in 1,000), suicidal thoughts, severe allergic reactions
- Red flags: Stop immediately if seizures, rash, or suicidal thoughts occur
This comprehensive guide provides detailed information about Mysimba side effects, their timeline, and management strategies.
What Are the Side Effects of Mysimba?
Mysimba combines naltrexone and bupropion, which may cause:
| Frequency | Side Effects |
|---|---|
| Very Common (≥1 in 10) |
Nausea, vomiting, constipation, headache |
| Common (≥1 in 100) |
Dizziness, insomnia, dry mouth, anxiety, fast heartbeat, sweating, rash |
| Uncommon (≥1 in 1,000) |
Hives, abnormal dreams, memory loss, erectile dysfunction |
| Rare (≥1 in 10,000) |
Seizures, severe allergic reactions |
Did You Know? Taking Mysimba with food and staying hydrated can reduce nausea. Avoid evening doses to prevent insomnia.
When Do Mysimba Side Effects Start?
Side effects typically follow this timeline after starting treatment:
| Time After Starting | Likely Side Effects | Why They Occur |
|---|---|---|
| First 3 days | Mild nausea, headache | Body adjusting to naltrexone's effects on gut receptors |
| Week 1-2 | Peak nausea, vomiting, constipation | Dose escalation begins (1→2 tablets daily) |
| Week 3-4 | Insomnia, dizziness, dry mouth | Bupropion's stimulant effects at higher doses |
| Month 2+ | Most side effects diminish | Body adapts to medication |
How Long Do Mysimba Side Effects Last?
Most side effects are temporary:
Typical Duration of Common Side Effects
- Nausea/headache: 2-4 weeks (usually resolve as body adjusts)
- Insomnia/dizziness: 4-6 weeks (often improve with stable dosing)
- Constipation/dry mouth: May persist but often manageable
Factors Affecting Duration
| Factor | Effect on Duration |
|---|---|
| Following dose escalation schedule | Reduces severity and duration |
| Taking with food | Shortens GI side effects |
| Alcohol use | Prolongs neurological effects |
| Individual sensitivity | Varies by metabolism |
Serious Side Effects
These require immediate medical attention (frequency cannot be estimated from available data):
- Suicidal thoughts or behavior
- Seizures (risk increases with higher doses)
- Severe allergic reactions (swelling, difficulty breathing)
- Serotonin syndrome (when combined with antidepressants)
- Angle-closure glaucoma (sudden eye pain/vision changes)
- Liver injury (dark urine, yellow skin/eyes)
Note: The risk of seizures is about 0.1% and increases with alcohol use, eating disorders, or sudden withdrawal from sedatives.
FAQs
Why does Mysimba cause nausea more than other weight loss drugs?
Naltrexone in Mysimba directly affects opioid receptors in the gut, which can trigger nausea. This is most intense during the first 2 weeks as your body adjusts. Taking tablets with food and drinking plenty of water can help significantly.
Can I take anything to reduce Mysimba side effects?
For nausea, your doctor may recommend anti-emetics like ondansetron. For headaches, simple analgesics (paracetamol) may help. Never take additional bupropion-containing products. Always consult your doctor before adding medications.
Do side effects mean Mysimba is working?
Not necessarily. While some side effects (like reduced appetite) correlate with therapeutic effects, others (like nausea) are just temporary reactions. Weight loss results typically become noticeable after 12-16 weeks, not side effect presence.
What should I do if side effects don't improve after 4 weeks?
Contact your doctor. Persistent side effects may require dose adjustment or discontinuation. Never adjust your dose without medical supervision due to seizure risks.
How long after stopping Mysimba do side effects disappear?
Most resolve within 1-2 weeks after stopping. However, some neurological effects (like changes in mood) may take longer. Serious effects like allergic reactions may require separate treatment.
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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. Talal is a GMC registered doctor and medical reviewer at Chemist Doctor. He ensures clinical accuracy and reliability across health content.
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: 18 November 2025
Next Review: 20 May 2026
Published on: 18 November 2025
Last Updated: 19 November 2025
