Drug also accounts for significant proportion of young kids' intensive care admissions.
Methadone, the medicine used to help heroin addicts kick their habit, is the most common cause of unintentional fatal poisoning from prescribed drugs among UK toddlers, finds research published online in the Archives of Disease in Childhood.
The drug accounts for more than half of all such deaths among young children and for a significant proportion of young kids' admissions to intensive care, the findings show.
In 2013-14, the National Poisons Information Service (NPIS) received more than 14,000 calls from healthcare professionals about suspected childhood poisonings, many of which related to prescribed drugs.
While severe side effects and deaths among kids from this cause are rare, certain medicines could be fatal for a toddler swallowing one or two adult doses.
To find out the most common causes of unintentional fatal poisoning from prescribed drugs among young children in the UK, the researchers analysed national data on childhood poisonings and on kids' hospital treatment and admissions to intensive care for unintentional poisoning between 2001 and 2013.
During this period, 28 children under the age of 4 died in England and Wales as a result of unintentional poisoning with a prescribed drug. Methadone was the culprit in 57% of these cases.
Between 2002 and 2012, some 201 toddlers were admitted to intensive care as a result of unintentional poisoning with prescribed drugs. The agents responsible were identified in 115 cases.
Benzodiazepines (sedatives) accounted for nearly one in five (19%; 22) cases, while methadone accounted for a similar proportion (17%; 20).
The researchers emphasise that the number of hospital admissions for pre-schoolers as a result of unintentional poisonings fell by more than 20% in England between 2000 and 2011. Nevertheless, severe and fatal poisonings associated with prescribed drugs continue to occur, almost all of which could have been avoided, they say.
But a major drawback to making further inroads into these figures are the limitations of the current sources of information, including the age bands and the circumstances they cover, and an absence of systematic data recording, the researchers point out.
"There is a need for robust and systematic recording of the medication involved and circumstances around all exploratory ingestions that result in significant harm to young children. This requires regular review to inform targeted public health interventions to avoid these tragedies," they write.
In 2013 more than 2 million prescriptions for methadone were issued to more than 140,000 adults.
It is an effective and useful medicine, but not without risk, say the researchers. "It is the second most common cause of drug related death in England and Wales after heroin/morphine," they say.
"In addition, those to whom methadone is prescribed, due to their underlying condition, are potentially least able to guarantee it will not be accessible to young children," they say, adding: "It is vital that before and during the prescribing of methadone, extensive efforts are made to ensure the safety of children who might be at risk of exposure."
Article: Severe and fatal pharmaceutical poisoning in young children in the UK, Archives of Disease in Childhood, doi 10.1136/archdischild-2015-309921, published 16 May 2016.