Goldman Sachs downgraded the Boston Beer Company and Constellation Brands due to sluggish beer sales and shifts in demographic tastes. As it turns out, younger generations are drinking more wine and less beer.

"As we explored back in 2014, we expected a cyclical rebound in total alcohol consumption post-recession ," wrote chief analyst Freda Zhuo in Monday's note. "The cause [for shifts in beer and wine market penetration] is younger groups shifting away from beer."

After the first hour of trade on Monday, shares of Boston Beer were down 5 percent, while shares of Constellation slipped nearly 1 percent.

Goldman not only suggests that young drinkers aren't consuming as much alcohol as previous generations did, but they also observed that millennials are trading beer for wine and spirits.

"We view the shift in penetration and consumption trends as driven by a shift in preferences in the younger cohorts," added Zhuo. "The youngest demographic (<35 year olds) overall penetration rates are not increasing. The 35-44 year old cohort shows a shift away from Beer to Wine & Spirits."

Nielsen panel data showed that beer penetration across the United States is 25 percent year to date versus 26 percent in 2016 according to Nielsen panel data. Wine and spirits' penetration was stable at 23 percent and 14 percent respectively.

Neither Boston Beer Company nor Constellation immediately responded to requests for comment.

Goldman now has a sell rating on Boston Beer Company and a neutral rating on Constellation Brands. Goldman lowered its overall US beer volume forecast to a decline of 0.7 percent in 2017.

"Despite the Boston Beer Company's commitment to turn around Sam Adams beer and Angry Orchard cider, we see no improvement in sight and see downside risk to fiscal year 2017 volume guidance."

Goldman lowered its 12-month price target for Boston Beer to $110 from $140, representing 20 percent downside from Friday's close.

Goldman trimmed its price target for Constellation stock to $210 from $212, representing 6 percent upside from Friday's closing bell.

"We remove Constellation from the Americas Conviction List and downgrade to Neutral from Buy, mostly driven by Constellation's price appreciation year-to-date and less upside potential to our price target."

But wine and spirits are not the only beer substitutes millennials are pursuing. Cowen recently adjusted its rating for Molson Coors, citing increased marijuana usage.

The report found that during the three most recent cycles of alcohol consumption there was a "notable inverse correlation with cannabis use." Cowen analyst Vivien Azer cited how during the 1980s and 1990s alcohol consumption fell 22 percent while marijuana use rose 18 percent.