My guess is that it's related to the reduction of prostate size... but that is a total guess.
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I'm waiting for this to return to normal before I begin using finasteride topically. I know the risk of sides is roughly the same since topical finasteride works by going systemic, but if I can see this side effect is in fact reversible I don't have a whole lot to lose by trying again with a slightly different route. It's a shame, because finasteride completely stopped my hair loss after ~7 mos.