Maree Mavis Crabtree is alleged to have killed her 26-year-old son Jonathan with an overdose of painkillers on July 19, 2017 in the family's home north of the Gold Coast before making a $125,000 insurance claim.
The 59-year-old has also been accused of attempting to murder her son in January of the same year.
Crabtree's defence counsel last week told the jury Jonathan might have accidentally or deliberately given himself too much medication.
The Brisbane Supreme Court jury on Monday heard testimony from Rebekah Millard, who started a relationship with Jonathan in 2012 that lasted 18 months.
"About eight months into the relationship with you he had an overdose  of drugs?" defence barrister Angus Edwards asked Ms Millard.
"Yes, he did." she said.
Ms Millard said Jonathan always had the prescription drug tramadol but never mentioned another painkiller called oxycodone.
The jury previously heard a pathologist determined the cause of Jonathan's death was an excessive amount of the drug oxycodone in his bloodstream.
Mr Edwards asked if Jonathan said he had taken the overdose as he "could not handle it anymore".
Ms Millard said she could not recall but agreed she had told a previous court hearing that Jonathan gave that explanation.
She agreed Jonathan had wild mood swings and always threatened to kill himself.
"When around friends he was the happy-go-lucky Jonathan but when it was just us it was a different story," she said.
Jonathan was a "troubled man" with abusive behaviours who had been charged with robbing a chemist, the jury heard.
A car crash in 2015 had left him with permanent injuries requiring physical care.
Crabtree lived in a house with Jonathan and his sister Tara.
Crabtree had said before Jonathan's death that he was "eating her out of house and home" and had the "brain capacity of a six-year-old," the Crabtrees' neighbour Vicki Inglis testified on Monday.
"Jonathan had ownership of the house and she could not get him out of there," Ms Inglis said Crabtree had told her during conversations between the car crash and his death.
Ms Inglis was the latest of multiple witnesses to testify that Crabtree talked about giving Jonathan drugs.
"Marie said 'I should put something in one of his syringes?'"crown prosecutor Caroline Marco asked.
"Yes. On a number of occasions over a number of months," Ms Inglis said.
Crabtree made the remarks while talking about seeing Jonathan shooting up drugs in his bedroom, the jury heard.
"Did she say the reason to put something in his syringe was to harm him or do him some harm?" Mr Edwards asked.
"No," Ms Inglis said.
The trial is due to run for another four weeks before Justice Martin Burns and hear from dozens more witnesses.
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