When I started taking the Herb Pharm adrenal tincture it decreased my milk supply. I cannot find the links now, but my research had lead me to believe it was the prickly ash bark. It coincided with my desire to wean anyways, so I took advantage of that.
I was nursing a toddler at the time, so we only nursed during the days at nap time and bed time. She had free access to the breast at night and nursed often. Maybe it wouldn't affect a mother that was nursing more often.