Sales of most medical marijuana products from Hydropothecary Corp. will resume after the Gatineau medical marijuana grower announced that it had recalled products affected by the discovery of trace amounts of an unapproved pesticide.

Hydropothecary announced the recall Tuesday morning, saying it was still searching for the source of the contamination at its facility in Masson-Angers, about 40 kilometres northeast of Ottawa.

The company voluntarily stopped all sales two weeks ago after a Health Canada spot check found the pesticide myclobutanil on plant samples.

Hydropothecary becomes the fourth medical marijuana producer in the past six months to recall products after testing showed the presence of myclobutanil and other pesticides not approved for use on cannabis.

Growing concerns about tainted medical marijuana prompted Health Canada earlier this month to order the country’s 43 producers to start testing for pesticides. Some companies, including Hydropothecary, had already begun such testing on their own.

It’s not known how many Hydropothecary customers have been affected by the sales halt and recall, as the company has declined to release those numbers.

However, the two-week sales halt was bound to be troublesome for the company and inconvenient for customers. Patients are only allowed to possess a month’s supply of medical marijuana at a time, or 150 grams of dried marijuana or the equivalent in oil, whichever is less. Any Hydropothecary patient who ran out of medicine during the sales halt would have needed to obtain a new doctor’s prescription to purchase from another producer.

The recall affects 14 lots of cannabis products sold between Feb. 1, 2016 and May 1, 2017. It was not clear how much dried bud and oil that represents. A company statement said lot sizes vary, but each lot contains only one strain and type of product. Customers who are affected are being contacted, the company said in a news release. 

Initial testing results indicate the contamination occurred during “older production,” the company said.

Company officials have been searching horticultural records from the nursery and garden centre that used to occupy the 65-acre site.

Hydropothecary produced its first marijuana harvest there in the fall of 2014, and has expanded rapidly since then. 

The recall announced Tuesday is classified as a “Type III,” which Health Canada defines as “a situation in which the use of, or exposure to a product is not likely to cause any adverse health consequences.”

In its search for the source of the myclobutanil, Hydropothecary sent 254 samples to independent labs for testing, including live plants, dried cannabis, cultivation equipment, soil, water, fertilizer salts and wall scrapings from growing rooms.

“We will continue to strive for the highest possible standards for all our products and to put patients first,” said Sebastien St-Louis, company co-founder and chief executive, in the news release.

St-Louis said it’s a “new and quickly evolving industry. For example, the detectable level of myclobutanil (0.01 parts per million) was only established last week by Health Canada.”

The level of pesticide in the recalled products was minute, between 0.01 and 0.08 parts per million, the company said.

Health Canada said that no level of myclobutanil is acceptable, because it is not one of the pest-control products approved for use on cannabis.

Myclobutanil has been used in the illegal cannabis industry in the U.S. and Canada to control mildew.

It’s sprayed on plants in the early stage of growth, said scientist Jonathan Page, president of Anandia Labs in Vancouver, which specializes in cannabis testing and research. The laboratory tests cannabis for some of Canada’s medical marijuana producers.

Myclobutanil is approved for use on some fruits and vegetables, but has not been proven safe when applied to a cannabis plant that is dried and smoked.

“Myclobutanil is a fairly non-toxic pesticide if used appropriately, but in this particular instance when it’s smoked there is a lot of concern about its health effects,” said Page, who is also a professor in the botany department at the University of British Columbia. “More study needs to be done.”

“The industry has been put on watch with these pesticides … but the vast majority, even the ones that have had these problems, will institute procedures to figure out what went wrong and to fix them,” he said.

Three other companies have recently had pesticide-related recalls — Mettrum (since taken over by Canopy Growth Corp., the parent company for Tweed in Smiths Falls), OrganiGram and Aurora.

Patients who bought cannabis products produced by Mettrum and OrganiGram have complained of getting sick, and three lawsuits are seeking class-action certification in the courts.

Health Canada says it has received 10 “adverse reaction reports” related to Mettrum products sold during the period covered by the recall, one report related to an OrganiGram product, and no reports about Aurora products.

Health Canada will also continue the pesticide spot checks it began in March when samples were taken from seven marijuana producers, including Hydropothecary.