Methadone-related deaths have increased between 1999 and 2005 (the most recent data I found). Poisoning deaths in the United States that mentioned the use of methadone rose from 786 in 1999 to 4,462 in 2005.
This doesn't necessarily mean that all 4,462 deaths were caused by methadone abuse alone--other narcotics may also have been involved in those deaths. However, methadone is thought to have played a role.
Most of the poisoning deaths (nearly 80%) were classified as "Unintentional poisoning with mention of methadone," which I would interpret as being another way to say unintentional overdose.
Patients enrolled in methadone maintenance have low rates of overdose, and therapeutic use of methadone is considered safe.
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