A doctor is set to give evidence about cyanide poisoning in the trial of two lovers who allegedly murdered a Melbourne husband in his sleep.

Sofia Sam, 33, and Arun Kamalasanan, 35, are on trial in the Supreme Court for murdering Sam Abraham at the Epping family home in October 2015.

It was initially believed Mr Abraham had suffered a heart attack but an autopsy later revealed he died of cyanide poisoning and also had a sedative in his system.

Dr Michael Burke, of the Victorian Institute of Forensic Medicine, conducted the autopsy and his report informed the prosecution case.

Prosecutors say Kamalasanan, working with Sam, poisoned Mr Abraham in his sleep.

"By sneaking into the house and pouring orange juice with cyanide in it into the mouth of the deceased," prosecutor Kerri Judd QC said in her opening to the jury.

Citing evidence from Dr Burke, Ms Judd said people swallow saliva involuntarily as they sleep and it was possible to slowly feed someone liquid while they were sedated.

The doctor is expected to give evidence at the trial on Thursday.

Sam and Kamalasanan have pleaded not guilty.

They knew each other from their college days in India and they reconnected when they both moved to Melbourne.

It's alleged they were having a secret affair and expressed their affection through passionate diary entries.

© AAP 2018