A report from the Institute for Safe Medication Practices lists several reasons for the serious and sometime fatal overdoses that have occurred when methadone is used to treat moderate to severe chronic pain.
ISMP points out that methadone differs from other opioids in a number of ways. For example, methadone remains in the body long after its analgesic effect has worn off. Also, a patient may not experience the full analgesic effect of methadone until 3-5 days of use, so it must be titrated more slowly than other opioids.
And a high degree of tolerance to other opioids does not eliminate the possibility of methadone overdose. ISMP cites two fatalities and a near fatality from prescribing too large a methadone dose for patients who had previously taken high daily doses of Oxycontin or Vicodin. Also, if a patient on methadone stops taking the drug for three consecutive days, the patient may lose tolerance for methadone and be at risk for an overdose if the usual dose is resumed.
Errors have also been reported because of confusion between methadone and other drugs with "look alike" names. In one report, a 17-year old patient with a traumatic brain injury received 25 mg of methadone BID instead of methylphenidate and suffered respiratory arrest.
ISMP also points out that errors can occur because of confusion between mL and mg doses. In one case, a patient had been taking 13 mg/day of methadone, which was prepared in the community pharmacy using a 1 mg/mL methadone concentration. When the patient was hospitalized, the attending physician assumed that the hospital carried the same concentration and prescribed 12 mL of methadone without specifying the dose in mg. The order was filled with a stock solution that contained 10 mg/mL and administered to the patient --- an overdose of nearly tenfold. Fortunately the patient vomited most of the medication and survived.
ISMP recommends a number of steps to help prevent these kinds of life-threatening errors. Here are some of them:
•When prescribing methadone for pain, avoid concomitant use of other narcotics, benzodiazepines, and sedatives, because these significantly increase the risk of an adverse event. Prescribe oral liquid doses of methadone in mg, never in mL alone, since several concentrations exist. Include the indication for use when prescribing methadone, to avoid confusion with methylphenidate. Specify the exact time(s) for administration. If the daily dose is taken in the evening one day and then in the morning the next day, this could lead to an overdose.
•When dispensing methadone, use commercially available methadone solutions to prevent compounding errors. Stock only one concentration of oral liquid methadone in the pharmacy, if possible. Accept orders for methadone only when the dose is prescribed in mg. Label all unit-doses with the exact dose, including strength and total volume if it is a liquid, along with the date and time for administration.
•When administering methadone, adhere to standard medication administration times. If a dose is missed, check with the physician before administering it later than originally scheduled. Make sure a pharmacist has reviewed the order before giving the medication to the patient.
•Remind patients to take methadone exactly as prescribed. Instruct them not to start or stop taking any other medications or dietary supplements without talking to their prescriber, because methadone interacts with many other drugs.
Instruct methadone patients to seek medical attention if they experience symptoms of overdose, such as slow or shallow breathing and extreme sleepiness, or symptoms of arrhythmia.
FDA Patient Safety News: April 2009
For more information, please see our website:
http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/psn/transcript.cfm?show=85#7
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