Monteith, 31, was found dead in his room at the Fairmont Pacific Rim on Saturday.
According to the B.C. Coroners Service, the Canadian actor died from a toxic mix of heroin and alcohol. Vancouver police have said there is no legal reason to continue with their investigation into his death.
But Toronto defence lawyer Robb MacDonald, who says he has defended alleged drug dealers brought in for questioning under similar circumstances, says it's routine for such probes to go deeper.
"If they were somehow able to confirm who the source was, and they charged that individual, even if it was a simple trafficking charge — I think they could lay on more charges, but even if they went only as far as to lay a trafficking charge — it would be what we call an aggravating aspect of the sentencing," MacDonald said.
"It's going to be an aggravating factor in sentencing that this was a guy who sold drugs who killed someone."
MacDonald says it wouldn't be difficult for police to go through Monteith's phone records or speak to neighbourhood informants to track the source of the drugs.
But Vancouver police Const. Brian Montague said at a news conference Tuesday that police won't try to charge the person who provided Monteith with the drugs.
"From the police perspective, the investigation has been concluded," he said.
"There are provisions in the Criminal Code, like criminal negligence, provisions like there's a duty to provide the necessities of life. Our investigation shows that Mr. Monteith was alone at the time he died. The factors involved in his death wouldn't be relevant to an investigation of that sort."
Police say Monteith was out with friends the night he died, returned to his hotel at about 2:15 a.m. PT and consumed the heroin alone.
Montague says using illicit drugs comes with an inherent risk.
"There's no law in Canada that where we can request charges against someone's that's provided drugs to an individual," he said.
Police say there's no reason to reopen the investigation unless new information surfaces.
Monteith had been candid about struggling with addiction as a teen, opening up about the experience in a 2011 interview with Parade magazine and saying he was "lucky to be alive."
He also told CBC's George Stroumboulopoulos how the arts helped him though those difficult times — and why he discussed them publicly.
"What I'm trying to do now with this whole 'I had a hard time' thing," said Monteith, is to tell people with similar difficulties "find something that inspires you, find something that you can get excited about, and that will become your new direction."
Monteith was best known for playing Finn Hudson, a football quarterback with two left feet who found more camaraderie in the choir room than on the football field, on the hit TV show Glee.
The show became an instant success when it debuted in 2009 and made celebrities of Monteith and the rest of the relatively unknown cast.