Medical and legal professionals, police officers and pot producers are among the dozens of witnesses lined up to field questions from MPs this week about public health issues related to the looming legalization of marijuana.

The House of Commons health committee is returning to Parliament Hill a week early, holding five full days of uninterrupted hearings beginning Monday.

Committee chair and Nova Scotia Liberal MP Bill Casey said the goal is to have a focused, concentrated approach that will likely amount to several months' of work in just one week.

"We have a lot to learn and a lot to listen to," he told CBC News.

Some of the key issues Casey will be keeping an ear out for from a public policy perspective will be around preventing the contamination of cannabis growing facilities, the four-plant allowance rule and a minimum 18-year age of access.

Conservative Health critic Marilyn Gladu said while the government's stated objective is to keep pot away from minors, allowing four plants in a household allows an "easy access opportunity."

She also wants to hear expert witnesses speak about how to maintain quality control while allowing homegrown weed. Gladu hopes next week's hearings will also drill down on the proposed limit of 30 grams for possession, which she said amounts to between 60 and 90 joints.

"That sounds like a lot to me for personal possession and we have to make sure that in legalizing this for personal use, we're not providing any kind of opportunity for trafficking activities to go on," she said.

The government plans to have legislation passed by July 2018.

Casey said the committee will likely make recommendations on how to improve and strengthen the legislation, and he did not rule out a potential suggestion to alter the age limit.

"If the committee comes to the conclusion that there's a better way to do this, or improvement, then we'll make those recommendations, and it's up to Parliament and the minister to decide whether to do them or not," he said.

Liberals 'rushing' process

MP Don Davies, a New Democrat from B.C. and a vice-chair of the health committee, accused the Liberals of rushing the hearings process, "cramming" in witnesses to dilute parliamentary and public engagement and debate around flawed legislation.

"I'm concerned they're trying to rip the bandage off and move to the next stage without getting really varied and diverse input from Canadians," he said.

"I think, for some reason, Liberals are anxious to get this through committee stage and out of the eye of the public and stakeholders very quickly, and I don't think that's wise or called for."

Davies said there are widespread concerns from provinces about the tight timeframe and the impact on law enforcement, health authorities and young people.

The federal government wants to pass a bill legalizing cannabis by July 2018. (CBC)

He sees "gaping holes" in the legislation and says if they aren't fixed, the bill will not meet the government's objective of keeping marijuana away from youth and out of the black market.

There are also issues around strict plain packaging that will limit competition, and the prohibition on edibles, creams, patches, tables will leave those sales to the illicit market, Davies said.

"At least one third of the market is going to be kept illegal, and moreover the one thing they are going to legalize is actually the most unhealthy way to ingest cannabis, which is by smoking," he said.

Ontario unveils retail plan

On Friday, Ontario became the first province to announce a framework around the sale and consumption of marijuana, which includes 150 stand-alone stores by 2020 and an online ordering service.

The province also plans to impose an age limit of 19, on par with its legal age to buy alcohol.

Along with the witness hearings, the Commons health committee has also received 99 written submissions.

The Canadian Medical Association repeated its concerns about health risks associated with cannabis, particularly in its smoked form. It urged the government to set the legal age at 21.

"Children and youth are especially at risk for cannabis-related harms, given their brains are undergoing rapid and extensive development," the brief reads.

Medical marijuana

The CMA also recommends that there be a single regime to regulate medical and non-medical use of marijuana, as well as a statutory review of the legislation after five years.

The NDP says restricting legal cannabis to smoking will leave a third of the product to the black market. (Ben Nelms/Reuters)

The Federation of Canadian Municipalities is asking for more clarity on municipal zoning isues and how revenue will be distributed. It's also looking for federal funding to pay for ongoing administration, training and public health and education.

"These costs will likely be significant at the initial implementation stages, and on an ongoing basis," the submission reads.

Special apartment provisions

Canadian Federation of Apartment Associations is calling for an amendment to the bill that would prohibit marijuana growing or processing in multi-unit dwellings, and in rented dwellings of any size. The organization says there are significant safety risks and insurance costs associated with growing marijuana in rental units.

The Cannabis Canada Association, which represents licensed producers, outlined its recommendations for meeting the government's goal of dislodging the black market, including a pricing and tax regime that remains competitive with illicit producers.

It says some branding and advertising is necessary to differentiate legal products and sellers and "to guide Canadians in their use of a new and unfamiliar product."

Casey could not say how long the committee hearings will go on, but predicted that this week's witnesses will generate new questions and more witnesses.