Breastfeeding mothers who drink alcohol do not produce as much milk for their babies, a US study says.
Drinking alcohol impedes breast milk release, experts warn
Researchers studied the effects of drinking the equivalent of two glasses of wine on 17 women.
The team, from Monell Chemical Senses Centre in Pennsylvania, found on days when the mother drank alcohol less milk was released
But UK experts said mothers need not worry if they enjoyed the "odd glass" of alcohol.
The report, published in the Journal of Clinical Endocrinology and Metabolism, said drinking alcohol disrupted the release of two key hormones - oxytocin and prolactin - responsible for milk production.
Author Julie Mennella said the findings disproved the myth that drinking alcohol was good for milk production.
"This information is important for women. If a mother is drinking alcohol just to improve the quality or quantity of her milk, she needs to know that there is no evidence to support this claim.
"Since breast milk is a critical form of nutrients to infants, it is important that women remain as healthy as possible during the breastfeeding stage."
During the study, the women, who all had babies between two and four months old, were given orange juice containing alcohol on one day and just orange juice on alternate days.
After drinking alcohol, oxytocin levels fell by 78% on average, while prolactin rose by 336%, causing the mothers to take longer to prodice the first drop of milk and to release less milk overall.
In the UK, many doctors and midwives advise women that drinking in moderation is fine.
Pam Lacey, chair of the Association of Breastfeeding Mothers, said mothers who enjoyed a glass of wine need not worry.
"I think if mothers are sensible about it, having the odd glass is fine. They should not be drinking excessively and feeding a baby."
And Rosie Dodds, of the National Childbirth Trust, added: "From the baby's point of view, breastfeeding is always best, and an occasional drink does not necessarily do any harm. But it is better to feed the baby first."
Dr Sarah Jarvis, of the Royal College of GPs, said there was no evidence to show that in small quantities a mother's consumption of alcohol harmed their baby.
But she added: "There will always be some women whose quality of breast milk will be affected by alcohol. If women want to guarantee the quality of their breast milk, giving up alcohol during breastfeeding is a small price to pay."