Nearly 20 laboratories across Canada have come forward to offer independent testing of marijuana after the federal government changed its laws last month, allowing consumers to screen the product – which is often sold as medicine – for contaminants and potency.

Previously, the public was banned from getting access to Health Canada-accredited labs to ensure the safety and efficacy of the drug, which the government plans to legalize next spring. The move comes after concerns were raised that the federal government has watched idly as unregulated marijuana dispensaries proliferated across the country, reaping huge profits in a booming prelegalization market, though some of the products would not meet federal health standards.

Globe Investigation: What's in your weed? We tested dispensary marijuana to find out (Subscribers only)

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An investigation by The Globe and Mail in July showed that one-third of the cannabis samples tested from nine storefront marijuana shops in Toronto would not pass a Health Canada safety test. Three of the nine samples contained high levels of potentially harmful bacteria that were beyond allowable health limits. One sample contained excessive amounts of mould and yeast, which can cause serious lung conditions.

Prior to the investigation, consumers who were worried about such contaminants, including people with compromised immune systems, or parents seeking to give extracts such as cannabis oil to children with conditions such as severe epilepsy or brain tumours, were prohibited under federal law from having it tested for safety.

Two weeks after the revelations, Health Canada sent an urgent memo to accredited laboratories across the country, which was obtained by The Globe. The memo asked whether they would be willing to start testing for the public under a new federal program, "in recognition that laboratory testing of cannabis is an important consideration for the health and safety of Canadians." The labs were asked to respond "urgently," in a matter of a few days.

The change now makes lab testing widely available to Canadians who seek it, due to a new exemption from the Controlled Drugs and Substances Act that previously prevented the facilities from possessing any amount of the drug brought in by the public. Anyone with a prescription from a doctor to purchase the drug from a regulated supplier, or who has a federal permit to grow it, can access the labs, provided the person has filled out the appropriate paperwork.

Though Health Canada considers cannabis purchased at the hundreds of dispensaries proliferating across the country to be illegal, the change effectively allows customers of those dispensaries – or operators of the dispensaries themselves – to test product as well, provided they have filled out the same paperwork, which is easy to obtain online. Consumers wishing to test unregulated supply from dispensaries would now be able to do so, because the labs don't have the ability to know where the products came from.

Jonathan Page, who runs Vancouver-based Anandia Labs, a federally accredited facility that has added its name to the list of labs, called Health Canada's decision to legalize testing for consumers "quite significant."

"The fact that they now allow testing is a big step forward," Mr. Page said. "Patients didn't have access to testing. So I think there was sort of a moral imperative to bring it in."

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Safety is a crucial issue for patients who use the drug as medicine. Several labs told The Globe they regularly turned away people such as the parents of severely epileptic children who administer cannabis oil to their child to quell seizures, and from relatives of elderly patients who use the oil to ease serious arthritic pain and other illnesses. Previously, labs were required to break the law if they wanted to assist patients who sought to ensure the products weren't mislabelled for potency or contained hidden contaminants, such as pesticides, harmful bacteria or mould.

Wendy Riggs, manager of lab services at M.B. Laboratories Ltd. in Sidney, B.C., said that before Health Canada's memo was distributed, there was uncertainty about what labs were allowed to do. Facilities such as M.B., which is also on the list, are dependent on the federal government for their licences, and fear angering regulators. "It's a relief," Ms. Riggs said. "I'm glad to have it clarified."

The lab that agreed to test the samples obtained by The Globe did so as a public service, but only on the condition it would not be identified. In July, Health Canada told The Globe it was not possible to allow labs to test for the public. However, the government quickly changed course in early August. Since then, 17 accredited facilities have signed up to offer the services. The roster includes a cross section of the sector, ranging from independent facilities that test pharmaceutical products for safety, to university laboratories, to labs that specialize in forensic work and research.

While a full battery of tests for chemicals, heavy metals, micro-organisms and other contaminants can run close to $1,000 a sample, the most relevant tests for patient safety can be completed for as little as a few hundred dollars or less.

Since the announcement, Ms. Riggs said there's been an increase of inquiries from groups of growers, or collectives, who are interested in pooling their resources and having their products tested. "I think it's a positive thing because it brings awareness of safety to patients and the people who are dependent on this drug."

Mr. Page said he's also seen an increase in phone calls and e-mail inquiries. The biggest concerns are potency levels and microbes, such as bacteria and other contaminants, which can be dangerous for those with compromised immune systems.

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"Micro seems to be something that people are bringing up, and it's a relatively inexpensive test compared to some of the other ones," Mr. Page said. "They're just trying to gain more understanding of what they're consuming as a medicine."