That's an interesting and substantial question. Can elevated prolactin, alone, cause what is in fact gynecomastia, or is it a matter of aggravating pre-existing (from puberty) previously unnoticed gyno, and/or inducing lactation in the absence of gyno?
I don't know the answer to those. I'd be surprised if elevated prolactin absolutely alone, no elevations in estradiol or progesterone, could cause gyno, but I don't know.
Aggravating the nipple area, even to the point of inducing lactation, and causing situations self-reported as gyno? That does occur sometimes with Deca.
I view it as a completely avoidable problem even without cabergoline or pramipexole, because:
1) Most who use Deca are doing so for no reason which cannot be better satisfied by another injectable
2) Where there's actually a point to Deca use, namely for joint issues, all it takes is a low dose such as 50-80 mg/week (the high end of that is really a matter of injection frequency and measuring convenient amounts in the syringe rather than an exact needed figure) and these problems don't seem to occur, assuming estrogen is controlled,
and I always recommend or write the substitution or reduced dose.
You are right on the progestin issue being another adverse quality of Deca. Pramipexole or cabergoline will do nothing for that but have helped the nipple-area problems anyway. Perhaps the combination of adverse effects (elevated prolactin and progestagenic activity) is a worse culprit than one alone.
Deca is a totally over-promoted steroid.