A two-year-old boy who died after taking his mother's methadone had been exposed to other drugs, a court heard.
Sally Dent, 32, and Shaun Binfield, 44, from Belper, Derbyshire, are on trial for the manslaughter of their son Riley Pettipierre and child cruelty.
Riley was found unconscious on 13 March after he took methadone which had been stored in his drinking beaker.
The court heard police also found evidence of heroin and cannabis in hiding places around the home.
On the second day of trial the jury at Nottingham Crown Court was told scientific tests showed traces of these drugs in strands of Riley's hair.
Prosecution barrister, Yvonne Coen QC, said that far from being a tragic accident, it was highly likely Riley had also consumed heroin and cocaine in the months leading up to his death.
She said: "Far from being a tragic one-off, it would seem there was something of a culture of neglect in this household concerning drugs.
"It can't be said either parent deliberately gave them to Riley, but he did come into contact with a cocktail of potentially lethal drugs. It was an obvious risk with very grave consequences."
Concluding her opening of the case Ms Coen added: "The death of any child is a tragedy. This death was wholly avoidable.
"It is the duty of a parent to look after their children. Riley's parents failed in the most serious way by not keeping him safe."
At an earlier hearing the court heard Ms Dent was a regular user of heroin and crack cocaine and been prescribed methadone to break her addiction.
She had been sleeping off the effects of the drugs at the family's flat in Belper when Riley reached out to the beaker on some bedroom drawers, the court heard.
Mr Binfield was watching television in another room and Ms Dent awoke to find Riley lifeless in the bed next to her, the hearing was told.
Post-mortem tests showed the toddler had enough methadone in his blood, stomach and urine to prove fatal.
Ms Dent and Mr Binfield deny the charges. The case continues.