Does drinking alcohol slow muscle growth?

Short answer

Yes, if you drink a lot. Drinking 1.5 g of alcohol per kg of body weight after exercise (over 2 bottles of soju for a 70 kg person) cut the rate of muscle building by 24–37%. But a third of that, 0.5 g per kg, made no difference to recovery, and 1–2 beers a day did not reduce the results of 10 weeks of training.

Does drinking alcohol slow muscle growth?
'Alcohol → blocks muscle building → lost gains' — does this formula hold?
A common belief says one drink after a workout wipes out that day's training. We checked how much alcohol interferes with muscle building, and how much you have to drink for it to matter.
Heavy drinking slowed muscle building
Even with protein, it dropped 24%
8 people drank 1.5 g of alcohol per kg of body weight (about 12 drinks) over 3 hours after exercise. Compared with a protein-only day, the rate of building muscle protein was 24% lower when they drank with protein and 37% lower when they drank with carbs. For a 70 kg (154 lb) person, that is about 2.2 bottles of soju.
Strength dropped more the next day
At 1 g per kg, strength loss went from the 20s-30s to the 40s (%)
10 people overworked their thigh muscles, then drank either 1 g of alcohol per kg of body weight (about 1.5 bottles of soju at 70 kg) or a non-alcoholic drink. 36 hours later, strength was down 26–32% without alcohol but 41–45% with it.
So what about a little?
5 g per 1 kg made no difference
The same team repeated the test with half the amount: 0.5 g per kg of body weight (about 3/4 of a bottle of soju, or about 900 mL of beer, at 70 kg). Strength recovery at 36 and 60 hours did not differ from the non-alcoholic drink.
One or two beers every weekday didn't block training results
Over 10 weeks, fat went down and muscle went up
72 people did high-intensity interval training 2 times a week for 10 weeks while drinking beer every weekday: 660 mL for men, 330 mL for women. Like the no-alcohol groups, they lost fat and gained lean mass (muscle and everything except fat). Gains in fitness and grip strength were not affected by alcohol either.
But heavy long-term drinkers had less muscle
A survey of 196,561 people: the more drinking, the steeper the drop
A survey of 196,561 adults found that muscle mass peaked in moderate drinkers and fell steeply with heavier drinking. At 160 g of alcohol a day (about 3.4 bottles of soju), muscle was 3.6–6.1% lower than in non-drinkers. When the same people were measured again 4 years later, there was no difference by how much they drank.
Bottom line: it's how much you drink that costs muscle
The formula only holds on heavy-drinking days
On days with more than 2 bottles of soju after training, muscle building slowed and strength recovered later. At or below 3/4 of a bottle for a 70 kg person, recovery did not differ, and 1–2 beers every weekday did not reduce 10 weeks of training results. Keeping the amount under that line on training days works.