A two-year-old boy slipped into a coma after being given a "potentially life-threatening" dose of a drug used in the treatment of epilepsy, a family court judge heard.
Judge Kambiz Moradifar was told that another child in the little boy's family suffered from epilepsy.
He concluded that either the little boy's father or the father's then-partner had administered the drug.
Doctors had told how the little boy had suffered two fits and needed to be put on a ventilator.
Blood tests showed that he had ingested a "very large quantity" of carbamazepine.
His father and then partner had denied administering the drug.
It had been suggested that another child in the family might had been responsible but Judge Moradifar said the evidence did not support such a scenario.
The judge said a feature of the boy's father's life was the use of "illicit drugs".
Detail of the case has emerged in a written ruling following a private family court hearing.
The judge, who is based in Reading, Berkshire, has not identified the family involved.
Social workers with welfare responsibilities for children in the family had asked the judge to make findings of fact.
Judge Moradifar said the little boy and two other children had been placed into foster care pending decisions on their long-term futures.