Killer dentist Colin Howell injected his lover with drugs to knock her out while they had sex and once feared he had overdosed her, a court has heard.
Hazel Stewart wanted to be unconscious during intercourse so she would not experience any Christian guilt, another former boyfriend of the mother of two told her double murder trial.
Trevor McAuley said Stewart had also warned him to steer clear of Howell, then her ex, because he had no idea "what he is capable of".
Stewart is accused of killing her husband Trevor Buchanan and Howell's wife Lesley in May 1991 and making police believe it was a suicide pact.
Her then lover Howell has already pleaded guilty to poisoning their spouses and leaving their bodies in a fume-filled garage in Castlerock to make it appear they had taken their own lives. She denies the charges.
The third day of the trial at Coleraine Crown Court also heard that in the period before the murders, Howell gave his wife sleeping tablets to knock her out so he could meet Stewart to carry on their affair.
Mr McAuley, who started seeing Stewart five years after the deaths and as her relationship with Howell was ending, told the jury she had told him that when she was involved with Howell he would come round to her house with drugs in a "floppy needle".
"He would administer this drug to her and she would pass out and she would really know nothing about it until the morning when she woke," he said. "On one occasion he almost overdid it and gave her too much and he was actually concerned if he was going to get her to come round."
Asked by Crown lawyer Neil Connor why Howell gave her the sedatives, Mr McAuley replied: "So he could enjoy sexual gratification with her without her feeling guilt of it while he was able to have pleasure."
Under cross-examination, Mr McAuley indicated the guilt Stewart claimed to have experienced was in relation to her Christianity.