A preliminary toxicology report shows that Justin Bieber tested positive for marijuana and Alprazolam, a key ingredient in the prescription medication Xanax, hours after he was arrested last week in Miami.

The toxicology report, released Thursday afternoon, shows that the star tested positive for both drugs hours after he was arrested in Miami on Jan. 23, under suspicion of drag racing under the influence.

According to the report, the star tested negative for a number of other drugs including cocaine and opiates. Miami Beach Police tweeted out a link to a local media report containing the test results Thursday afternoon.

When Bieber was initially arrested he told Miami police that he had consumed alcohol, smoked marijuana and had taken prescription medication. Xanax is commonly prescribed for anxiety.

The news comes just hours after Bieber turned himself in to Toronto police Wednesday night to face an assault charge in connection to an alleged altercation with a limousine driver last December.

The Canadian pop star arrived at Toronto’s 52 Division, amid a throng of screaming fans, media and officers. He was released roughly two hours later.

Around the same time he was at Toronto police 52 Division headquarters, a short video message was posted to Bieber’s Instagram, announcing that his latest single "Confident" had been released.

"Go get it. Go look at it. Go comment," he says in the message.

Toronto police confirmed on Twitter Thursday that the video was not shot while Bieber was in police custody.

Early Thursday morning Bieber's father tweeted a photo of his son sleeping in bed with his little brother. "Safe and sound #homesweethome," Jeremy Bieber wrote.

Sources tell CTV Toronto that after spending the night at the Hazelton Hotel, Bieber travelled to his hometown of Stratford Ont., to spend time with family and re-focus his attention on work.

The assault charge stems from an incident outside a downtown Toronto nightclub in the early hours of Dec. 30, 2013, police said in a statement issued Wednesday.

Police say a limousine picked up a group of six people outside the club around 2:50 a.m. On route to a hotel, an altercation occurred between one of the passengers and the limo driver, police allege.

They say a man allegedly struck the driver “on the back of the head several times.”

The driver stopped and got out of the limo to call the police, but the suspect left before officers arrived. None of the allegations have been proven in court.

In a statement issued Wednesday night, Bieber’s representatives said he is innocent. They said they would not comment further, as the matter is now before the courts.

Toronto-based criminal lawyer Boris Bytensky, who is not involved in this case, said that while the maximum penalty for a simple assault charge is six months, he'd be "shocked" if the star spent a day in jail.

"I expect that it would proceed by way of summary conviction, which is our equivalent of a misdemeanor," he told CTV News Channel.

Bieber has already been charged in Miami with driving under the influence, resisting police without violence and driving with an expired licence. He has pleaded not guilty to the charges and was released shortly after his arrest last week.

And earlier this month, police in Los Angeles searched Bieber's Calabasas mansion following a complaint from a neighbour whose home was allegedly egged.

During the police search, a member of Bieber's entourage, Xavier Smith – who goes by the stage name "Lil Za"-- was arrested on suspicion of cocaine possession.

Following his Miami arrest, a petition was submitted to the White House asking for Bieber to be deported back to Canada.

Bytensky said despite the allegations Bieber's facing stateside, he likely won't be deported given the "value" the star adds to the community as a performer and public figure.