Occasional drinking while breastfeeding has not been shown to have any negative effects on breastfeeding babies.
Every person is different but on average, a woman weighing 68 kg (150 lb) will have a blood alcohol level 0.01% one hour after consuming one drink, and 0% after 2 hours. This same average woman will have a blood alcohol level of 0.05% one hour after drinking two drinks, 0.03% after two hours, 0.02% after three hours and 0% after four hours.
If your baby feeds when your blood alcohol level is 0.01%, your baby will be drinking milk with 0.01% alcohol. If your baby feeds when your blood alcohol level is 0.05%, your baby will be drinking milk with 0.05% alcohol. Those are both extremely small levels of alcohol. For example, plain apple juice contains 0.06-0.66% alcohol (depending on the brand).
There is the possibility that drinking large amounts of alcohol could decrease your milk-ejection reflex (letdown). This could reduce the amount of milk your baby gets in the few hours after you have consumed alcohol. Research has shown this reduction in milk volume to be small (less than 10%) and will not be experienced by all women.
Drinking large amounts of alcohol can cause drowsiness, sleep problems, increased crying, decreased weight gain, and possible developmental delays in your baby.