I have argued before that penile shrinkage is not a symptom of PFS only but is also the normal mammalian response to finasteride and is in this case perhaps a function of total exposure to the drug. In my personal experience, I had penile shrinkage before I got PFS. This website publishes some anecdotal evidence about penile shrinkage from men taking dutasteride and finasteride for benign prostate hyperplasia. The men also report sexual side effects so PFS cannot be ruled out, although, as we know, ongoing sexual side effects are not the same as PFS.
