Closing arguments wrapped up in Montreal on Thursday, with Quebec Superior Court Justice Brian Riordan taking the case under deliberation.
While the trial began in March 2012, the legal proceedings actually date back to 1998.
The plaintiffs include an estimated one million Quebecers who argue the companies are liable because they knew they were putting out a harmful product and hid the health effects of tobacco.
The suit involves separate groups of plaintiffs — some who became seriously ill from smoking and others who said they couldn't quit.
The defendants are Imperial Tobacco Canada Ltd.; Rothmans, Benson & Hedges Inc.; and JTI-Macdonald. They argue the dangerous health effects of tobacco have been common knowledge for decades and that there was no conspiracy to conceal anything.
Initially the lawsuit was valued at $27 billion and included 1.8 million Quebecers, but a 2013 ruling changed the definition of who could qualify.
Mario Bujold, director-general of a Quebec council on tobacco and health, called the case historic.
"The whole question of the responsibility of tobacco companies with regard to their products is at the heart of this case," he said Thursday.
"If a court acknowledges the responsibility of the companies, it's clear that moment will be a game-changer."
Bujold said that could eventually lead to changes at the social level or even in legislation.
Tobacco companies have denied publicly that smoking carried risks, he added.
"As far back at the 1960s, they have done everything to prevent people from knowing the truth," Bujold noted.
The trial heard from 76 witnesses, while some 27,000 documents were filed as exhibits, including many confidential company memos and studies that showed people didn't know or understand the health risks relating to smoking.
The industry has argued people knew about the risks of smoking and that the products were sold legally and with federal government approval.
"People knew about the health risks associated with smoking for many decades and the federal government who licensed and enabled us to sell those products knew about those health risks for many decades,'' Imperial Tobacco Canada spokesman Eric Gagnon said earlier this year when asked to sum up his firm's arguments.
Lawyers for the firms did not speak to reporters on Thursday.
The case is distinct from civil suits launched by several provinces to recoup health-care costs from smoking-related disease, but many of the arguments in those cases overlap.
All provinces have passed laws that allow them to go after so-called Big Tobacco for health-care costs stemming from smoking-related disease and most have filed legal actions to that effect.