The partner of a man who was "deliberately poisoned with opiates" has been charged with his murder.

Wendie Dent, 59, was arrested by detectives on Monday morning following an 18-month investigation into the death of her partner, 63-year-old Adelaide man David Lawrence.

He was found dead in his Adelaide home in 2015 of an overdose of drugs that police allege was deliberately fed to him.

At the time of Lawrence's death, Dent had been living with him for 18 months when his will was changed in the weeks leading up to his death.

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According to South Australian Detective Superintendent Des Bray, Lawrence was a healthy man.

"We do know that several days before he died he became very unwell and ceased all contact with the outside world," Detective Bray previously told the ABC.

"Of significance, he received calls on his mobile phone, even on his birthday, and those calls went unanswered."