Retin A which was very common at one time, has declined in popularity over the years.

It is not really a simple treatment.
Here is a link with some common side effects:
http://www.ncbi.nlm.nih.gov/pubmedhe...s#side_effects

Some is absorbed into the body.
And it is related to Accutane which is one of the most toxic drugs on the market today.

This site has FDA Medwatch reports tabulated.
The low numbers reported reflect the unpopularity of this drug today. When it first came out, there were many more people rushing to use it, but that has changed.

http://www.drugcite.com/?q=tretinoin
This list of reports show far more toxicity than other drugs on the market in my opinion. So your question about PN is only a very small part of what might happen. Stay alert, therefore.

Retin A has been found to cause severe birth defects in pregnant women to the fetus. This paper explains now the drug affects neural cell development:
http://www.ncbi.nlm.nih.gov/pubmed/21934132

Drugs typically showing this toxic potential to new cells, can affect adults as well. For example Thalidomide which causes severe defects in developing fetuses, is also neurotoxic to adults. It is used
today to treat multiple myeloma, and also causes a PN in those patients. Adult nervous systems, are always repairing themselves, from free radical and other damages, so drugs like this which suppress dividing cells, could be potentially damaging to adults.
This is why Retin A was modified, to the new Retin A micro product.