Barley, a main ingredient in stout, contains a polysaccharide that increases prolactin, a milk-production hormone. Oats, another stout ingredient, are also thought to promote healthy milk supply, as is vitamin-B-heavy brewer’s yeast

That is a total myth. And it’s alcoholic, so you definitely don’t want to be drinking that while breastfeeding!

It is apparently true. But you are better off having brewers yeast. Chemist warehouse sell this in tablet form and I found it helped my supply.