Canagirl, there is general information about possible side-effects and contra-indications of Viagra/Sildenafil here; http://www.drugs.com/mtm/sildenafil.html .

The paper is very interesting but it was a study of mice with a mutation which makes them susceptible to diabetes and diabetes-associated PN.

Whether or not Viagra will help people with diabetes-associated PN or people with PN which is not linked to diabetes remains to be seen.