OTTAWA -- Growers on the cusp of Canada's nascent marijuana industry are bracing for Thursday's long-awaited Liberal legislation on legal pot, which sources say is expected to require the newly unshackled drug to be sold only in plain, brand-free packaging.

The prospect of plain packaging, which tobacco manufacturers are also opposing, has pot producers warning the federal government that they won't be able to compete with the black market without some form of branding.

Health Minister Jane Philpott spoke to a Senate committee Wednesday about requiring plain packages for tobacco products -- a measure that was also recommended by the federally appointed task force on marijuana.

In an interview with The Canadian Press, Philpott would neither confirm nor deny that the new bill would require plain packaging for legal marijuana, offering only that there are indeed public health lessons to be drawn from the experience with tobacco.

Prior to the bill's introduction, a number of prominent producers lobbied the government to resist the notion of plain pot packaging.

Seven companies-- Tilray, Tweed, Mettrum, CannTrust, Green Organic Dutchman Holdings, RedeCan Pharm and Delta 9 Bio-Tech -- wrote to Philpott and Prime Minister Justin Trudeau to warn about the potential consequences.

"Without branding and in-store marketing collateral, it will be difficult to educate consumers about the products they are buying and help them differentiate between products," they wrote.

"Brands also ensure accountability, encouraging producers and retailers to provide quality products and support in the new market."

Members of the medical community will also be watching to see if Ottawa proceeds with a recommendation to limit sales to those over 18 -- something Trudeau seemed to endorse last year when he described it as a reasonable compromise.

The Prime Minister's Office declined to say if he still feels that way, saying only it would "legalize, strictly regulate and restrict access" to cannabis in a careful way to keep it out of the hands of young people and to prevent criminals from profiting.

For its part, the Canadian Psychiatric Association has warned about the mental health implications of cannabis for young people, and recommended an age limit of 21, as well as quantity and potency limits for those under 25.

Early and regular cannabis use can affect memory, attention, intelligence and the ability to process thoughts, said CPA President Dr. Renuka Prasad, and exacerbate the risk of psychotic disorders and other mental health issues among those already vulnerable.

The driving purpose of the Liberal government's plan is to address Canada's "very high rates" of cannabis use among young people that are among the highest rates in the world, Philpott said.

Criminalizing cannabis has not deterred its use by young people, she added, noting other products including alcohol and tobacco are available with restrictions for legal consumption despite known harms.

Marijuana is a perfect example of the look-before-leaping approach the Liberals took to their 2015 campaign promises, Conservative MP and leadership candidate Erin O'Toole told The Canadian Press in an interview Wednesday.

He sided firmly with those provinces who want the federal government to pick up the tab for additional law enforcement costs sure to result from the new landscape.

"I think the federal government has an obligation because they started this move," O'Toole said.

O'Toole said he would rather see a "hybrid" version of decriminalization that would allow police to ticket users, while retaining the ability to lay a criminal charge at their discretion.

He also disagreed with his running mate Kellie Leitch, who said Tuesday she would reverse Trudeau's pot plan if she became leader and eventually prime minister.

"To suggest ... after the horses are all out of the barn that we are going to get them all back in the barn, I also don't think is a pragmatic policy statement.