Like any medication, methadone has real side effects — most manageable, some worth flagging to your provider directly.
Common side effects:
- Constipation (often persistent throughout treatment, not just early on)
- Sweating
- Drowsiness, especially early in treatment or after a dose increase
- Weight gain, reported by some patients
- Reduced libido or sexual side effects
Less common but important:
- QT prolongation — a heart rhythm effect that’s dose-related; see our dedicated article [link]
- Drug interactions — several medications affect methadone levels; see our dedicated article [link]
What’s often mistaken for a side effect but isn’t: Feeling stable and functional without euphoria is methadone working as intended at an appropriate dose, not a sign it’s “not strong enough.”
When to call your provider rather than wait it out:
- Significant, persistent drowsiness beyond the first couple of weeks
- Any signs of an irregular heartbeat, fainting, or palpitations
- Side effects significantly affecting your daily function
Medically reviewed by [pending]. Sources: SAMHSA TIP 63, FDA prescribing information for methadone.