I developed PD for a short time in high school and my best friend had it as well. She went to the dermatologist and he told her to use fluoride free toothpaste. We both switched and we both had it clear right up.
I recently developed it again about two months ago. I was in Boston, (where they pump fluoride into their water, but I did not know this until later) I ran out of my usual fluoride free toothpaste and borrowed my roommate's (which contained fluoride) and had a huge break out.
I have been trying everything since, and although apple cider vinegar, yogurt and turmeric masks, a biotin pill and a probiotic made huge improvements on the PD, it was still not going away.
I did more research and found out that WINE contains high amounts of FLUORIDE! Even though I have read that you should give up alcohol to get rid of it, I figured one glass of wine here and there wouldn't hurt.
The second I stopped drinking wine completely it began to subside and now is almost gone.
After doing lots of research, it was such a relief to figure out why it wasn't going away. I also think it largely helped that I moved from a city with lots of fluoride in its water system to one without.
(I also found in addition to wine, that beer, soda, tea, processed cereals, juice, teflon pans and salt among many other foods and beverages contain high amounts of fluoride, so make sure to do your research and cut these things out.)