Concentrated alcohol is suspected of being added to bottles of Stella Artois beer, Canada's food watchdog says.

The Canadian Food Inspection Agency said Thursday that tampering is suspected in six 330-millilitre bottles of Stella Artois beer with best before dates of November and December 2005, and production codes of L32343T and L35243G.

"These beer bottles appear to have been tampered with in that the contents have been removed and replaced with an ethanol, which is basically a concentrated alcohol solution," said Garfield Balsom, a CFIA spokesman.

As these beers' expiry dates have passed, they should no longer be for sale in the marketplace. The advisory does not apply to Stella Artois cans and draft beer.

Consumers and servers at restaurants and bars are urged to check Stella Artois beer bottles for best before dates and production codes.

The CFIA also noted that bottled beer with two neck labels glued over each other should not be consumed.

The suspected tampering is believed to have affected beer sold at restaurants and bars in Toronto and Kamloops, B.C.

No incidents involving beer products sold in stores have been reported.

Vomiting has been linked in some cases to the consumption of the beer, the CFIA said.

"Because of the concentration, if someone consumes this they will immediately react," Balsom said. "That's what's happening and with the complaints they took a swallow of this beer and immediately reacted."

The CFIA and Labatt Breweries of Canada are working with the police in this investigation. Balsom said police believe it was a random act.

"The issue is still under investigation, Toronto police are involved and it appears that it's just a random act of tampering at this point, no motive or rationale, just a random act that we can understand at this point," he said.

Anyone with information about the suspected tampering should contact the local police department.