Quoted, thought it was a pretty good read:
Milk Thistle also known as St. Mary's Thistle (Silybum marianum) has a long history in the western herbal tradition as being a galactagogue i.e. a stimulant of milk production in lactating women. Until recently there was little evidence that milk thistle was a galactagogue and this application of milk thistle was folkloric.
For some reason unknown to me the Italians began investigating milk thistle in relation to lactation in 2004 with the publication of Effects of Silymarin, a Natural Hepatoprotector, in Periparturient Dairy Cows by Tedesco et al. They found that milk thistle did indeed increase milk production, in cows at least.
Di Pierro et al (2008) prompted to some extent by Tedesco et al (2004) (we know because they cite that study) examined the effect of milk thistle on lactating humans. They found that milk thistle boosted milk production in lactating human mothers by a whopping 85%(compared to placebo 32%).
But how does milk thistle produce increased milk production? Capasso et al (2009) sought to determine how milk thistle acts as a galactagogue. Using female rats they found that milk thistle raises prolactin levels and that this appears to be the mode by which milk production is increased. They also found that administering Bromocriptine (a dopamine D(2) receptor agonist) attenuated the hyperprolactinemic effect of milk thistle.
The last piece of the puzzle (i.e. the Capasso study) was published only a few months ago so this is a very recent result and conclusion. It is only one paper and the result is yet to be reproduced but the prior two studies point in the same direction so it can't be dismissed entirely.
I suggest that if your are prone to gynecomastia or have experienced gynecomastia on-cycle -- especially gynecomastia with lactation -- that you refrain from using milk thistle as a liver protectant. If the Capasso study is confirmed by further studies then milk thistle would be a herb entirely unsuited to men in general.
Thread: Milk Thistle Raises Prolactin