The crash occurred on Highway 427 in the early hours of Aug. 5, 2012.
It left Jayantha Wijeratne, 49, and his 16-year-old daughter, Eleesha, dead. Antonette Wijeratne, the wife and mother of the deceased, survived the crash but spent months in hospital recovering from her injuries.
Prosa's blood-alcohol level was nearly twice the legal limit, according to an agreed statement of facts that have been entered in court.
But Prosa has testified that he doesn't remember the event and that he believes someone had slipped a drug in his drink at a nightclub before the crash.
His lawyer, Alan Gold, contends the Crown has not proven beyond a reasonable doubt that Prosa knowingly ingested whatever intoxicant was responsible for him consciously getting behind the wheel and driving the wrong way on the highway.
The defence has also said it has been unable to test for any drugs that may have been in Prosa's system at the time because the blood sample leaked before it reached the lab.
The judge is expected to rule on the defence's contention that Prosa's right to a fair trial has been compromised because of the lost blood evidence.
Prosa has pleaded not guilty to all 12 charges, which include two counts of impaired driving causing death.
Closing arguments in the trial continue in court on Thursday afternoon.