Substance guides

Methadone side effects: What patients should know

Published June 1, 2025 · 7 min read · Updated April 2026

Reviewed for accuracy by licensed clinical professionals.

Methadone is highly effective for opioid use disorder but has a side effect profile that requires monitoring, particularly cardiac effects.

Common side effects

Sweating (often persistent and significant). Constipation (chronic, requires active management). Weight gain. Sedation (especially during dose stabilization). Sexual dysfunction (reduced libido, erectile dysfunction). Dental problems. Sleep disturbances.

Serious side effects

QT prolongation (heart rhythm abnormality that can cause fatal arrhythmia). ECG monitoring is recommended, especially above 100mg daily. Respiratory depression (risk increases with dose and combination with other sedatives). Severe constipation leading to bowel obstruction in rare cases.

Long-term considerations

Methadone maintenance may be long-term or lifelong. Bone density changes have been reported. Hormonal effects (reduced testosterone in men, menstrual irregularities in women). Despite side effects, the benefits of reduced overdose death and improved functioning overwhelmingly outweigh the risks for most patients.

Authoritative sources

This article references guidelines from: NIDA · SAMHSA · CDC

Frequently asked questions

What are the worst side effects of methadone?

QT prolongation (heart rhythm risk), excessive sedation, and severe chronic constipation are the most concerning.

Does methadone make you gain weight?

Weight gain is common due to improved appetite, reduced activity from sedation, and possible metabolic effects.

Can methadone affect your heart?

Yes. Methadone can prolong the QT interval, creating arrhythmia risk. ECG monitoring is recommended, especially at higher doses.

Disclaimer: Informational only. Not medical advice. SAMHSA: 1-800-662-4357.