Milliliters (mL) vs. Milligrams (mg) in Methadone Dosing
No, 6 milliliters (mL) is not the same as 6 milligrams (mg) of methadone. These are different units of measurement that cannot be directly equated.
Understanding the Difference Between Volume and Weight
Milliliters (mL) measure volume, while milligrams (mg) measure weight. These units cannot be used interchangeably when discussing medication dosing:
- Milliliter (mL): A unit of volume
- Milligram (mg): A unit of weight/mass
Methadone Concentration and Conversion
For methadone specifically:
- Standard oral methadone liquid concentrate contains 10 mg of methadone hydrochloride per 1 mL 1
- Therefore:
- 6 mL of methadone liquid = 60 mg of methadone (6 × 10 mg/mL)
- 6 mg of methadone would be contained in 0.6 mL of the standard solution (6 mg ÷ 10 mg/mL)
Clinical Implications and Safety Concerns
This distinction is critically important for patient safety:
- Confusing mL with mg could lead to a 10-fold dosing error with methadone
- Such errors could result in significant overdose or underdose
- Methadone has a narrow therapeutic window and high potency with a conversion factor of 4.7 compared to morphine 2
- Methadone overdose can cause respiratory depression, QTc prolongation, and death 3
Best Practices for Methadone Dosing
- Always specify both the volume (mL) and the concentration (mg/mL) when prescribing liquid methadone
- Use metric units exclusively for oral liquid medications to reduce dosing errors 4
- Be aware that methadone has a long half-life (8 to >120 hours) and requires careful dosing 2
- For patients on methadone doses >100 mg/day, cardiac monitoring may be necessary due to risk of QTc prolongation 3
Common Pitfalls to Avoid
- Never assume a 1:1 relationship between mL and mg for any medication
- Avoid using household measurements (teaspoons, tablespoons) for medication dosing
- Be particularly cautious with methadone due to its potency and variable pharmacokinetics
- Remember that methadone's long half-life increases risk of accumulation and delayed toxicity
Understanding the difference between volume (mL) and weight (mg) units is fundamental to safe medication administration, particularly with high-risk medications like methadone where dosing errors can have serious consequences.