A 17-month-old baby had cocaine and cannabis in his body when he was beaten to death by his mother's boyfriend, a court has heard.
Finley Thomas was exposed to drugs by his mother and her partner before he was killed, jurors at Cardiff Crown Court were told.
His mother Chloe Thomas, 25, was addicted to illegal drugs and diazepam during her infatuation with Sean Buckley, 28.
A port-mortem examination of Finley's body showed he had the same drugs in his system, the BBC reports.
Roger Thomas QC, prosecuting, told Cardiff Crown Court: "Allowing him to suffer drug contamination is evidence of a lack of care and a lack of concern amounting to neglect."
Buckley is accused of murder, while he and Thomas are charged with causing child cruelty.
The court previously heard that paramedics arrived at Finley's home on 23 September, 2014, and were told by both defendants he had fallen down the stairs.
He died in hospital the following day.
However, the Crown argues the child's injuries were no accident.
In his opening, Mr Thomas told the court: "The catastrophic injuries sustained by Finley were a result of a deliberate and vicious act by Sean Buckley.
"And to avoid the consequences of his own violent conduct, he has persisted in the deceitful manufactured account and has been assisted by the lies and support of Chloe Thomas.
"Most falls down the stairs...do not cause subdural haemorrhages."
The court previously heard how a garden chair was used to kill Finley.
Mr Thomas played a DVD to jurors to show how the meshing on a collapsible chair found in the couple's garden matched with marks on the baby's scalp.
Thomas and Buckley deny the charges against them.
The trial continues.
Additional reporting by PA
