Michael had drug cocktail, court told.
The 43-year-old singer was found slumped at the wheel of his Mercedes in Cricklewood, north London, on October 1 last year.
Prosecutor Andrew Torrington told Brent Magistrates' Court that tests showed a therapeutic quantity of an anti-depressant in the singer's system as well as the illegal dance club drug GHB - a class C substance.
Cannabis was also found in his blood, but the prosecution do not maintain that this was the cause of his intoxication.
Michael Grieve, for Michael, who did not attend yesterday's hearing, said, "It is the defendant's case that his condition was caused by taking prescribed drugs."
He said there was a strong body of opinion which said that a sleeping drug was the most likely explanation for his condition.
He also told the court it was common ground that GHB could be present in the blood without any illegal substances having been taken.
Mr Torrington told the court that while GHB was a banned drug, its effects could be created by drugs which are not controlled, including a drug called GBL.
Mr Grieve agreed with District Judge Katherine Marshall that the level of the singer's unfitness could be viewed as high.
He said there was no dispute as to the description of the singer's condition as being semi-conscious at the wheel.
But in terms of considering sentence, he told the district judge, "One looks at the unfitness, not the cause of it. Whatever the level is, that's all you need to look at."
He later added, "We will not be contending on behalf of the defendant that the fact that his condition was due to the consumption of prescribed drugs combined with tiredness as opposed to any illegal drugs is a mitigating feature of the offence.
"Subject to that, we will be inviting the court to sentence him on the basis of plea - after hearing full mitigation."
District Judge Marshall adjourned sentence until June 8. The case was previously adjourned to hear from expert witnesses.
But the judge said it was now unnecessary to hear from them.
She said, "It is clear that it is not necessary for me to go any further in this expert evidence - I will sentence on the basis that counsel has asked me to."