Three major grocery chains are pulling romaine lettuce from their Canadian stores, in response to a public health warning about possible E. coli contamination.

Loblaw and Sobeys have both said that they have stopped selling romaine lettuce nationwide. Metro only operates in Ontario and Quebec but has also removed it from all their stores. Walmart Canada is also removing romaine lettuce products from its shelves, and it says customers who have already purchased them can return them to the store for a full refund.

“Out of concern for the health of employees and customers, Sobeys Inc. temporarily stopped the sale of all Romaine lettuce products across its national store network until further notice,” reads a press release from Sobeys.

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“Out of an abundance of caution, Loblaw Companies Limited is recalling and removing from store shelves across the country all romaine lettuce products,” reads a statement from Loblaw.

The Public Health Agency of Canada issued a warning Tuesday afternoon, telling people in Ontario and Quebec not to consume romaine lettuce.

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There have been 18 cases of E. coli in Ontario and Quebec, according to the warning. Six people have been hospitalized, one with serious complications. No one has died.

A recall has not been issued at this time.

“Contaminated romaine lettuce may still be on the market, including in restaurants, grocery stores and any establishments that serve food,” according to PHAC.

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The U.S. Centers for Disease Control also issued a warning Tuesday, telling all Americans to refrain from eating romaine lettuce and retailers and restaurants not to serve it.

“Consumers who have any type of romaine lettuce in their home should not eat it and should throw it away, even if some of it was eaten and no one has gotten sick,” the CDC said.

The agency also advised U.S. consumers to wash and sanitize refrigerator shelves and drawers that have contacted romaine lettuce.

Symptoms of E. coli can include nausea, vomiting, headache, mild fever, severe stomach cramps and watery or bloody diarrhea. According to PHAC, most symptoms end within five to 10 days, but you should contact a health care provider if symptoms persist.