Left Hand Brewing is recalling at least 20,000 cases of its most popular beer because a foreign yeast got into the mix and put way too much fizz into bottles of Milk Stout Nitro.

Spokeswoman Emily Armstrong said the beer is safe to drink. However, when it is “hard poured” — basically flipping the bottle upside down into a glass, as is recommended — the brew likely will overflow. “It is definitely not to our specifications,” she said.

The affected bottles, distributed in 37 states including Colorado, carry “best by” dates from Oct. 16 to Feb. 28. The bottles also were included in the Longmont-based brewer’s Mountain Mixer variety packages with codes ranging from 130B to 242A.

Consumers are asked to return the beer to the point of purchase, however Armstrong said the company still is working out replacement details. The process will be tricky because alcohol regulations vary from state to state. She said if customers are unable to return the beer, they should contact Left Hand directly at lefthandbrewing.com/contact/#Feedback.

“It’s definitely going to be challenging,” Armstrong said. “But we don’t want to put up a roadblock. People can reach out to us. We would certainly rather restore people’s faith in us, rather than turning our back on them.”

It will be four to six weeks before replacement bottled Nitro Milk Stout is available, Armstrong said. The beer still is available in kegs, she said, because the packaging procedure is different. “Consumers can still find Nitro Milk Stout on tap.”

Left Hand has begun dumping beer that was in production and is scouring its production lines, Armstrong said.

Left Hand production chief Chris Lennert said in a letter to customers posted at lefhandbrewing.com/contact/nitro-recall that the foreign brewing yeast, which looks just like Left Hand’s house ale yeast under the microscope, came in contact with the house ale yeast and got into bottles.The foreign yeast kicked off a secondary fermentation in the bottles “and is causing an increase in CO2,” he wrote. “The resulting effect is changing the gas qualities of the beer and causing the beer to overflow and disrupt the customary nitro cascade.”