Liam Darcy died last Wednesday, three days after swallowing methadone, a synthetic heroin substitute, at his grandmother's home in Solihull, West Midlands.
Police confirmed yesterday that preliminary tests on Liam's body showed he had fatal levels of the drug in his system when he was rushed to Birmingham Children's Hospital last Sunday.
At an inquest which was opened and adjourned yesterday, Coroner Richard Whittingham said that the child was unconscious when he was taken to Birmingham's Heartlands Hospital after failing to wake up last Sunday. He was later taken to the Children's Hospital in Ladywood but died after three days.
His mother, Nicola Darcy, a veterinary nurse, and her partner, Christopher Williams, were understood to be too upset to attend the hearing.
The incident happened at the Darcy family home, to which Ms Darcy is understood to have returned recently following a disagreement with Mr Williams. A West Midlands spokeswoman confirmed that one person at the house in Hobs Meadow was a registered methadone user.
Detective Inspector John Jones, the man leading the inquiry, said: "I can confirm that a number of persons have been arrested in connection with drug-related offences, and they are currently on police bail. There is a full, on-going investigation into the circumstances surrounding this tragic case."
Last month, five Lancashire coroners spoke out about the "tremendous naivety" among the public about the number of drug overdoses. One of the five, Andre Rebello, had talks with Jack Straw, the Home Secretary, asking him to reduce the weekly amount of methadone that chemists could give addicts because it created a market for the drug.
In February last year, an inquest recorded a verdict of accidental death in the case of Daniel Fitzpatrick, a 15-month-old baby who died after drinking methadone that belonged to his mother, 19-year-old Sinead Fitzpatrick.
A three-year-old boy who made the same mistake last month Glasgow survived after being rushed to an intensive-care unit at the children's hospital in Yorkhill, Glasgow. After doctors successfully fought to save the boy's life, Sam Galbraith, the Scottish health minister, said: "This underlines the great need for parents to keep all drugs well out of reach of children. Lessons must be learnt."