Members of the Victoria Police homicide squad have interviewed a Melbourne doctor for a third time over the medical treatment of a cancer patient who died in 2005.

Steve Guest, of Point Lonsdale in Victoria, was dying from oesophageal cancer when he was given the drug Nembutal by Dr Rodney Syme.

Dr Syme said he gave him a "sufficient quantity to give him control over the end of his life".

Dr Syme, 79, is the vice-president of Dying with Dignity Victoria and has long campaigned for a change in the law to allow voluntary euthanasia.

In his statement to Victoria Police, Dr Syme said Mr Guest died 16 days after he provided the medication.

He also said Mr Guest had been clear in his statements that he wanted control over the end of his life.

"I did not encourage or incite Steve Guest to end his life," he said.

Mr Guest died of oesophageal cancer in 2005.

"I did not intend that Steve Guest end his own life."

Dr Syme said he had provided the medication to "palliate his extreme psychological and existential suffering".

He said he had made voluntary statements about the treatment of Mr Guest "deliberately, in order to clarify the position of doctors" who provide end of life treatment.

"The lack of clarity in the law results in patients suffering unnecessarily or doctors acting clandestinely to support their patients," he said.

"Since the Parliament of Victoria declines to clarify the law in this matter, my statement is offered to allow the criminal justice system to determine the matter."

Dr Syme said police indicated that they were unlikely to prosecute him because of a "lack of evidence".