By what mechanism does Risperidone swell breast tissue? There has been much talk of the anti-psychotic drug Rispeirdone causing un-natural breast tissue growth as well as galactorhea (milk production). Especially in young men and boys. What is the mechanism of this drug that is causing this? Is it actually introducing some amount of female hormones into the male's body or is just causing increased production of them?

Risperidone prevents dopamine from acting on the pituitary gland by blocking dopamine type 2 receptors. One of dopamine's effects on the pituitary is to suppress prolactin production. In patients taking risperidone, prolactin production thus increases and prolactin levels in the body are higher than normal. In younger males, this can result in abnormal growth of breast tissue and even milk production.

See e.g. Melkersson, K. I. (2006). Prolactin elevation of the antipsychotic risperidone is predominantly related to its 9‐hydroxy metabolite. _Human Psychopharmacology: Clinical and Experimental_ , 21(8), 529-532.